The 7011-250 is the smallest.. a little pizza box about the size of a SparcStation 20 or Indy. It will run AIX up to 5.1 but pretty slowly. 4.3 or lower is a better here. I guess a ThinkPad 860 would be smaller if you don't mind serial console for AIX 5.1.
In a slightly larger desktop case, the 43P could be a decent choice. These come in various configs, the 140 (166MHz-2xxMHz 604/604R), 150 (up to 375MHz 604R), and slightly more rare 240 (same CPU modules as the F40, usually around 233MHz 604R). These will all do up to AIX 5.3 with reasonable expectations for the clock speeds.
There's a 40P in a similar case but it's a dog, stay away unless you want to run OS/2 or WinNT for PPC

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The B50 is a 2U rack server with similar insides to the 43P-150. These are somewhat plentiful and all around decent machines.
All of the above machines are 32-bit if that matters to you. Everything past this point, save 1U servers and blades from the past decade, is much much larger. Step up to a 44P, a rather large tower, and you're now in 64-bit land but still limited to AIX 5.3. These are pretty common, but be prepared for some shipping costs.
If you want to do AIX 6 or 7, you'll need a POWER4 or greater system. IntelliStation POWER is a good choice, but you may be able to score a server for cheaper if you look diligently. For some reason sellers mark these way up compared to other hardware. Maybe because they look sexy, or because IBM didn't shit on their UNIX biz? Not sure, but the supply isn't limited compared to any other gear.
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SGI Fuel, Indy R5k
IBM RS/6000 7006-42T, 7011-250, 7012-397, 7012-G40 (upgraded to 4x 200MHz PPC), ThinkPad 710TE vintage tablet, ThinkPad T400, various System X, NetVista 2800
Sun Ultra 27 Xeon Quad Core 3.20GHz, Sunblade 2500 Silver, SunFire V445
HP c8000
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