The slab-R is on the skimpy side relative to the rest of it, but not too much. With framing fraction downrating of the R23/R38 and up-rating of EPS for ~40F center-foam temp it's hard to say exactly what the U-factors of the SIPs will be at +15F outdoor temps, but let's give it a WAG, eh?

To hit R23 means you have 5.25" of R4.2/inch EPS inside the SIP for about R22, with the OSB skins adding the other ~R1. When it's +15F outside and 65F inside, the mid-EPS temp is about 40F, and it's average performance will be about R4.5/inch, for (5.25 x 4.5=) ~R23.5, add the R1 for the OSB and you're at ~R24.5. But something on the order of 10% of the surface area is thermally bridged by structural elements like top/bottom plates, window & door framing that runs about R1.2/inch for the most commonly used framing species. The 10% framing fraction reduces the average to about R19.5 nominal once the So the U-factor at your 99% outside design temp is about (1/R19.5=)U-0.051

Similarly the roof SIPs have about 8.75" of foam, but have skylight/eave/ridge structures probably running about 7% of the total surface area reducing the average R to about R34, for a U-factor of (1/R34=) U- 0.029.

You'll need to measure the actual surface areas of each to run the calculation- your description doesn't provide quite enough info for me to run it. But it's the same as what I'll show for your windows & doors:

Assume an interior temp of 70F, exterior 15F, for a 55F temperature difference

The slider has (6' x 6'8" =) 40 square feet, of U0.20, for a heat loss of (U0.20 x 55F x 40'=) 440 BTU/hr

The casement has (2'4" x 3'6" ) 8.16 square feet of U0.20, for a heat loss of ( U0.20 x 55F x 8.16'=) 90 BTU/hr

The awning has (3'0" x 2'4" =) 7 square feet of U0.20, for a heat loss of ( U0.20 x 55F x 7' =) 77 BTU/hr

The skylight has (2'6' x 4'0" =) 10 square feet of U0.45, for a heat loss of ( U0.45 x 55F x 10'=) 248 BTU/hr

Total window & door losses add up to (440 + 90 + 77 + 248= )

__855 BTU/hr__.

So, figure out the total roof area in that room (less the skylight), and use U0.029 x 55F x [area] to come up with the roof losses.

Then figure out the total exterior wall area, and use U 0.051 x 55F x [area] to come up with wall losses.

Ignore the losses out the slab- they'll be "in the noise" on the other errors.

Add them all up, and that'll be about what to expect for the heat loss at design temp. In really crude guesstimations, assuming you have (19' + 17'= 36' of exterior wall that averages 11' in height, that's 396 square feet, less ~55 square feet of window & door leaves ~340' of U 0.0.51 wall, for:

U0.051 x 55F x 340'=

__954 BTU/hr__
Figure the relatively low angle roof is just a bit more than the 19' x 17' room dimensions for ~325' square feet. We'll ignore the skylight area, and figure the roof losses are about:

U0.029 x 55F x 325'=

__518 BTU/hr__
Add it all up, you're at a mere 2300 BTU/hr or so, thanks primarily to the triple-pane glazing performance. Add a 10% fudge factor for air infiltration etc, and you're still ballparking around 2500-2600BTU/hr.

Assuming even a

__10F__ delta-T on the air you're moving vs. the air returning, that's (0.18 x 10F=) 1.8 BTU per cubic foot of air. To meet a 2600 BTU/hr heat load that's 2600/1.8= ~1450 cubic feet per hour or (1450/60 minutes per hour=) ~25 cubic feet per minute.

That's really all you'd ever need to heat that room.

Oversizing woodstoves in tighter better than code homes is becoming a common problem, since it turns the room with the stove into a sauna unless it's a high thermal mass stove (like soabstone), and you build hot, but intermittent fires that are allowed to burn down to nearly nothing.

Even the smallest mini-splits would heat & cool that room, so don't go any bigger than a 3/4 ton. A typical 3/4 ton mini-split would still have well over 10,000BTU/hr of heating capacity at +15F, but since they are modulating systems with between 3:1 and 5:1 turn-down ratios, it would still track the load. IIRC both Sanyo and Panasonic (Matsushita) use Fujitsu compressors & refrigerant valves in their mini-splits, but I don't have strong confidence in that recollection. They are both name-brand manufacturers with good reps, and the installer & local tech/parts support are the most critical aspects to pay attention to. A typical 3/4 ton Panasonic is rated ~14K in heating mode at +5F, with a pretty-good 10.5 HSPF (at 47F, full compressor & blower speed.) :

http://s3.pexsupply.com/product_files/E9NKUA-submittal.pdf
The older 3/4 ton Sanyos were something of a dog on heating efficiency (HSPF less than 8, barely legal), but I haven't seen a newer 3/4 ton from them.:

http://www.sanyohvac.com/assets/documents/submittal/09KHS71_submittal.pdf
Some of their bigger mini-splits test a lot better, but would be oversized beyond all reason, but there's no reason to go with anything that tests under HFPF 10 &/or SEER 18 for a single head mini-split heat pump these days. Mitsubishi, Fujitsu, and Daikin together hold down something like 3/4 the total US market, with Mistubishi being the sales leader, if not the efficiency or feature leader. The AOU/ASU 9RLS2 Fujitsu is probably the price/performance/distributor-service sweet spot for 3/4 ton mini-splits in many markets- with lots of installers familiar with the unit, excellent 12+ HSPF, and 27+ SEER. The hardware runs about $1500 from internet sources.

Where and how you mount the compressor unit matters, since you do get hit with the occasional blizzard. Bracket mounting it on an exterior wall above the anticipated peak snowpack depth, protected by roof overhangs is best-practice. Don't count on eaves to protect fully- under the rake of the roof is better, or you could end up in a situation like this, despite 2' overhangs on the eaves:

Most manufacturers have customized bracket mounting packages- but even if they don't, it's still possible to make up your own. This is what Fujitsu's bracket looks like: