it's hard for me to give any size. what size is comfortable for you to work on?
smaller than a lap quilt would to me be something about 3 ft by 4 ft maybe?
I'm a miniaturist so I am more comfortable with working on something like 14" x 14" or slightly larger.
your pattern will help to determine the size too
They can be square or rectangular like a table runner or octagon etc
Anything goes!