Is this an okay stitch length and size for padding the lapel?

It's hard for me to judge this close up and there may be some magnification in the image, but it looks OK. The stitches might be a bit long, but it depends on the desired effect. The smaller and more numerous the stitches, the firmer and more shaped the lapel or collar. Some tailors place stitches of different sizes and densities in different places if they want firmness but I've seen many lapels and collars that had the same stitch length and density throughout. I think the first book I read, by Roberto Cabrera, suggested starting with 3/8" stitches.

I've seen photos of lapels with stitches that long. Considering how much you've done I wouldn't redo it, especially as I believe you have a deadline.

The key thing is to hold the lapel after the first two or three rows of stitches so that you're building a roll into it. The lapel should not lie flat when laid on a table. The other thing is to pick up only a thread on the underside so one sees only rows of small depressions, not stitches.