I have 2 lines for supply heating in my basement ( my basement is about 500 sq fit)the duct are 6" d. what size the open for return has to be for this area , thak you for your help

A small 6" X 10" return would be fine. You could cut this into the return duct and add a grill if nessesary...

Are you sure you want a return in the basement? Do you have sufficient fresh air for combustion? Are the basement walls insulated? Return must be away from the furnace. This will raise the humidity in the house when A/C is running.