Although no hands-on experience with the pro520xl, I would expect it to be a pretty decent fully manual unit. Still what you could call a "compact" class radio in my opinion. However, if you can find one or wait for amazon to ship one, the pro520 clone by Luiton (the LT-298) could be a better choice for some people:
https://www.amazon.com/dp/B086YRZKJP/
The Luiton clone retains manual gain but adds optional automatic squelch (can still use manual squelch if preferred) and uses a 6 pin mic which has channel changer buttons right on the mic itself. Whether or not that is of any interest depends on how well they understand their needs, where this radio will ultimately be mounted, and how they will be using it on long trips. Unfortunately the LT-298 lacks ANL so that's a bit of a drawback, if the ANL on the pro520 is any good (again, no hands on experience with either). Just figured I would point out that option since the pro520 came up.
Another "compact" option that I have seen but not yet played with in-person is the Midland 1001LWX. It sounds good on paper, has ANL and weather, RF gain. Might be worth looking at. Having weather is nice, you can get sick of driving for hours and hearing nothing on CB since CB world is so dead in some places, at least with weather channels you can hear *something* out of the radio you paid money for and took the time to install
But since you said:
I need to find the best and smallest radio I can.
I don't know if you really mean that, but there are some options in the sub-compact class where the speaker volume starts to fall off because of tiny speaker in a tiny chassis but there are REALLY small radios such as
President Bill FCC [Channel change on mic, scan, weather, Manual Squelch & Auto Squelch, ANL+NB+Hi-Cut (can attest that this works in an electrically noisy car), no RF gain]
Radioddity CB-27 [No weather, no ANL/NB, has manual and ADJUSTABLE automatic squelch (via menu no knob), RF gain adjustable (via menu no knob), channel changer on mic, has scanning]
Cobra 19 MINI is another sub-compact unit that has RF gain adjustment via menu, but no ANL/NB, etc.
If you have room for something Pro505XL or Luiton LT-298 sized those will almost certainly have better speakers and be able to put more controls on the unit rather than slightly buried in menus. Again, depends on how well they understand how they will be using it. For me having the channel changer on the mic itself is huge because of where my radio ended up being mounted. ASQ is kinda nice and always optional, all radios have manual squelch that is a given. ANL/NB and even the Hi-Cut are pretty nice if your car emits gobs of EMI like mine does. I suspect a lot of incoming static can be reduced on something like an LT-298 if you use a noise filter on the power line and use the gain accordingly, but I haven't gotten my hands on one to try.
Everybody has slightly different needs and wants, 2-3 decent radios out there for almost any budget and application. Personally I hope to eventually collect them all so I have more to say about the options. Right now I'm just laying out all the things I've looked at but hardly used, except for the P. Bill in my car and my 980SSB that I use as a base station. If you have room the 980SSB is awesome...