If you get a 15" 5E3 it's not going to be much smaller than a 5E5A. A 5F4 isn't much smaller than a 5E5A as mentioned previously. It's about 3" shorter but the same width approx.
Get a neo speaker for your 5E5A and turn it into a 5E5 which is sort of like a two-channel deluxe with 6L6s. I'm doing that to mine right now.