We use a "bedside commode" to go over the toilet. However, the one we have has a small seat. Any suggestions for one with a larger seat?

I have seen commodes with soft cushion padded seats on eBay.

There is a commode called "Bariatric commode" ask wherever you got the one you have if they can order a bariatric commode.
This is what I used for my Husband and it was more to his size. (6' 4' and 250)
I even got extra PVC pipe to extend the legs a bit more than they normally would go.

Check for bariatric commodes, they are much sturdier and usually have better seats.

Looks like Amazon sells a nice selection of extra wide large bedside commodes, that should work for your mom. You may want to check them out. You probably can find them at your local medical supply store as well.