Large or small postcards? return the same?

Hi.  Anybody have any opinions on larger vs smaller postcards all
things being equal.

Would the larger one 5x8 instead of 4x6 get a better return?

I'm trying to decide if it's worth the extra budget?
There must be some rule of thumb from some advertising concept?
The only thing I can think of is that it's more space to tell your
story?

Any opinions?

Dan