Sizing a water heater for radiant heat

[zzip7]

I am planning to install a radiant floor heating system on one floor of my home (that floor is about 760sq.ft.).
I would like to use a domestic water heater, so that it will provide hot water for the floor and hot water for regular use around the house. I know that this is possible, but I don't know how to choose the proper size of water heater.
My current heater is a 40 Gallon natural gas heater, it has a 35 Gallon Per Hour recovery rate, but I don't think it's powerful enough.
ALSO: I should mention that this radiant floor system will not be used to HEAT the house, only to keep the floor warm.

Here a diagram (I will only have one zone though)
http://www.radiantcompany.com/images/muni.gif

THANKS!