can i use a small glass jar to freeze the dmt, or does it need a big bowl to form the crystals better?

You could freeze precip in any suitable container, but you’ll want to consider the ease with which you’ll be able to remove sticky, powdery, gooey stuff from it. My favorite is a square Pyrex baking dish with a well fitting lid. Curves aren’t your best friend in this situation.

I cut corners on a recent extraction and got a cheap round pyrex and a good part of the last couple yields will likely never be harvested.

Cut corners, double entendre?

Couldn't the leftover stuff be dissolved in a solvent and precipitated in the aforementioned square pyrex? Perhaps as a part of a future extraction, or in some solvent to be used for a re-x?

I have corner buildup in my rectangular pyrex dish, I'll be dissolving in ethanol to make a changa with.

If the container is used over and again for further extractions then the remaining crystals that you can't scrape out could be regarded as helpful in terms of acting as nucleation points for subsequent crystal formation and/or helping saturate your naphtha. They're not lost in perpetuity either way. Besides, you can make the effort and remove them with a relatively benign solvent such as ethanol, acetone or IPA and recover the remnants that way, or wash them out with a bit of vinegar and chuck them into the next extraction if you don't want to do a mini A/B. Pretty much as hedge said^.