use a counter current air exchanger. Hot inside air goes out, cold outside air comes in and the heat is transferred to the incoming cold air, saving the heat.

The size depends on the number of changes in the air needed, which depends on the people load, the combustion load, heating method etc

follow links on these pages.

http://en.wikipedia.org/wiki/Heat_exchanger

What is the height of the building?? What is the location? Ambient conditions and indoor conditions, number of air changes are few parameters needed to suggest the requirement of heat recovery system. I however based on your design calculations if you feel that the exhaust is considerable portion of the fresh air then heat recovery is recommended.

Consult Local Building Codes, SMACNA and NEBB standards. Some of the other responses list info you will need to plug-in.

If the outgoing air flow is less than the incoming air flow, the building will inflate...

or explode, ever hear of "fat building syndrome"?