That's plenty of room. Depending on the codes, which vary per state, there are some minimum requirements. For example- In CA you are required to have a minimum of 9" of non-combustible on each side of opening and 12" from opening to hearth. You don't need to go much larger to create a nice design....but of course that depends on what style you have in mind and what your aesthetic tastes are. There are many design styles and options vary depending on traditional design, modern, transitional etc. If you like modern, then you might prefer a larger box opening, but if your style is traditional, 36"Wide might be sufficient.

I would suggest finding a few fireplace design pics that you like. Use these to show the builder what you are thinking so you can determine if that's the size you want.

Good luck!

It's definitely not too large and you probably wouldn't want a fire box any smaller. It leaves you 18" on either side which is nice and balanced. However, you could do larger possibly. As SH Interiors said above, pull some pics of what you like and go from there.