What is the best size for 8 or 10 dwarfs then. And I was going to place a sea grass bed on one side no other live stock.

Normally I'd say 5 to 8g would be ideal.
However, I don't have first hand experience, but have read many times over on seahorse.org, that seagrasses need a mature deep sand bed, with high intensity lighting. You will have to take some time to develop a mature deep sand bed but from what I remember reading it may take a year or so. It's not like having macro in the tank, much of which can grow without any sand bed.
You will also have to provide hitches in shaded areas for the dwarfs to hitch on as high intensity lighting is not something they would be used to.

It is too bad that you live in London Ontario. I'd get you to come and help me set one up. I was just there last week.