It depends on a couple factors.
A smaller working area means less material used (i.e paint), but unless you have a fairly small needle size you will have to sacrifice detail. A larger working area is the opposite. It all comes down to personal preference and what you want to achieve or practice for techniques .For instance illustration paper is a great support for practice and finished pieces. It has a low cost and can be a little more forgiving then a hard surface when comes to absorbing paint. But then again too much paint and the paper will buckle. Not too mention you can't scratch textures into it without damaging it.
All supports have there ups and downs along with the different sized canvas'.
I like to use illustration paper that is a 9x12 size because of
1)low cost of paper
2)not a lot of paint is needed for projects
3)what ever you can do small you can do large
4)I use a lot of templates that i print and cut out from my printer ( which the paper is 8x11).
5) illustration paper is generally a multi medium paper (ie acrylic, ink, marker, pencil)
Anyways it comes down to personal preference,what techniques you want to practice and what you want to achieve in the end.
Hopefully this helps take care.