You need to do a heat load calc, then sizing is easy - compare the BTU/hr/ft at your supply temperature for the brand and style of baseboard you want, then just buy that. You'll be most comfortable when it isn't greatly oversized, as constant heat is more comfortable and ecconomical than when it is cycling on and off with long periods in between. But, then if the room is allowed to cool, you won't have enough to warm it up fast. So, there are compromises.
Heat load calcs are on-line if you do a little searching.
You need to know the worst case design temp, or make compromises. If you design it for -10, and it gets to -20, you'll never reach the desired set point.