5cu.ft would for sure give more flexibility with internal space and potentially more taps down the road if needed. I sometimes wish I had more room inside, but don’t have the drinking power in my house to support more than 2 taps at a time. Definitely worth the effort. Your son will be happy...
I built a small keezer from a 3.5cu.ft freezer. It is snug, but can fit two ball lock kegs and a 5lb CO2 tank inside.
The collar is 9" tall for keg clearance and lined with 2 layers of foam board insulation.
Made a small wooden platform to fill the 7" gap next to the compressor bump.
The total...