Sizes for Pastel Paintings

When I first began painting in pastels, I painted whatever size the painting turned out to be.  I thought that was how artists painted.  Soon, however, I discovered that my paintings required custom framing and that was expensive.  Little by little I discovered that some artists painted for particular frame sizes.  That is what I do now.

But what sizes?

Here's a small painting (5x7) that I started yesterday.

This could easily be done as a larger painting.

What painting sizes do you prefer?