Civic EG

I was wondering if i do a full exhaust on my d15b7 will it bit on a b18b when i do my swap ? and what size piping is optimal for both ? would 2.5 work ?

It's all dependant on how much your motor is putting out. if it's a stock motor. 2.25" piping all the way would be ideal.. For heavily modified NA motor 2.5" is ideal. If your boosting 2.5-3.0" is needed!!

Ok so 2.5 will be ok on the d15b7 in the meantime...but will the exhaust bolt up to a b18b?

as long as you use a test pipe or start the exhaust at the cat i should be fine