Is there a formula I should follow when sizing a new hydronic baseboard heater? I'm remodeling a bathroom in a 100 year old house.
You need a Manual-J or IBR type calculation of the heat loss to know how many BTUs you need to pump in there.
Then you need to know your water temp from the boiler.
You need to know the approximate flow rate in that loop.
You need to know where in the loop (unless it's a micro-zone with it's own thermostat), and calculate what the water temp coming into that baseboard is based on flow & amount of radiation that precedes it.
Then you can look at the manufacturers' spec on how many BTUs/ft you can expect to get out of their baseboard at that temp.
Without the heat loss, water temp, flow & other radiation you're just hackin', stabbin' in the dark.
This guy has something of a hydronics primer online, if you want to take the time to actually design it rather than hack away at it: http://www.heatpro.us/