Does this frame come with glass or another synthetic substitute for glass?

The "glass" is plastic and really reflect light very badly. Seeing the puzzle displayed inside is very difficult to see when there is any light reflecting on it.

Is the frame 24x30 or is the mat that size as shown in the picture?

The frame is 24x30. A smaller puzzle can be framed to use the mat.

Do you have to glue the puzzle before framing it?

If your puzzle is the same size as the frame, probably not. My puzzle was 1/4 inch too long, and a friend cut it with a utility knife without destroying the puzzle. I didn't glue it, and it's snug. But if the puzzle is smaller than the frame, I'm guess that any movement or jarring could knock it apart, The instructions tell you to glue it. I didn't, but I'm happy. Be sure to measure your puzzle before ordering. It must be EXACTLY 24x30 or smaller.

does this come with a plexi glass? thats a very important decsription that should have been indicated.

Yes it does. In the product description it says it has a Clear styrene front that protects the image from dust. It's not real thin either. I've used 6 of these for 1000 piece buffalo puzzles that are 26.75x19.75in. and it works great (comes with white/black full reversing backing for smaller puzzles). Just check the product for shipping damage upon receiving it because they are of composite construction and can be damaged quite easily (I've had a couple that were not in ‘good’ condition). They were replaced quite quickly with no issues (Free shipping and they picked up the damaged ones at my door [UPS] ). I always glue both sides of the puzzle. Pic side first. Then wait ~24 hrs after application so it's easy to flip over and won't stick to work surface.I’ve been using puzzle guard glue (can be found at walmart) with amazing results. I haven't tried other glues/laminates. This is My go to frame for puzzles.