I just built an 1800 sq ft. workshop with infloor heating. I use a NG boiler supplemented with 60 vacuum solar tubes. I really like the dustless infloor heat.
Use this heat loss calculator
http://www.builditsolar.com/Referenc...s/HeatLoss.htm
It will give you a good idea of what size heating unit you will be looking at.
What size/type pex pipe do you have?
Make sure that you get a compatible glycol.
Mitch