The well pump control box is about 4 1/2" inches wide by about 7" and around 4" deep so it sounds like it WILL work.

If you are in a rural area, you might contact your local well guy as they replace these things ALL the time and end up throwing them out. My guy gets a whole new control box and if a customer's is bad, he swaps out the lid (that contains the electronics like relay and start cap, etc.) so he ends up with old lids and new boxes. I am using the 1/2 to 1 horse pump enclosure (the smaller one).
Hope this helps.