You basically have to stay within a 300' limit on any radiant loops or you won't get enough flow. In most cases that will mean running 3/4" to purpose-made manifolds, and splitting out the multiple 1/2" loops. In an 800 square foot zone that will be at least 1200' of half-inch PEX, and an absolute minimum of four sub-loops on the manifolds, but going with somewhat shorter loops but more of them reduces the pump power necessary to make it all work. Keeping the lengths of the sub-loops identical is critical for getting the same flow through each loop too.
Read up on it before diving in- most radiant heating designers are using application-specific software tools to get it right, including pump specs, water temp, etc. Getting it "almost right" or "good enough" is certainly possible when hacking at it, but it won't always.
Every good heating design starts with a room-by-room heat loss calculation, from which you can work out the radiation/output required. Odds are you've been able to run your baseboards at 140F-150F and still have enough heat getting into the room at an outside design temp of ~ 20F(?)
Unless you want to become a junior radiant heating designer, it's probably better to PAY A PRO to do the radiant design, specifying everything from the heat transfer plates to the manifolds to the pumps, mixers, hydraulic separators,etc, even if you do the installation as sweat-equity. Do not substitute freebie radiant designs from web-vendors- they end up being more expensive and less efficient than radiant done right by a competent designer. A grand or so spent on a good design will save that much in excess/inappropriate hardware and a lot more in frustration level from having to re-plumb stuff to get it to work well.
Is your boiler one of the Contender models (sloping domed top) or one of the re-skinned Peerless Pinnacles (cube-shaped)? IRRC with the Contender there is no outdoor reset built in to the controls, whereas the li'l cubes do, which can make things easier to tweak in sometimes.