A nonbinding rule of thumb is to keep the width of your stills about twice the width of your render. That way, you've got some wiggle room for pan and zoom.

The problem is that jpegs have to be unpacked to uncompressed data sizes, and that takes up a huge amount of available memory. Irfanview has an excellent Batch Resize function, and you just as well save as PNG which is reportedly easier on Vegas resources.