Wow, people actually check my profile? Lol.
Arena 3, go to Engines -> Management -> UCI and there will be a hash size control there. You can also try to change the parameters for the specific engine by right clicking its name (You know how when an engine is thinking it shows a lot of analysis and above it Engine name, hash size used, then UCI, then current depth?). There should be a configure option for the specific engine.
The biggest issue with hash size and Arena for me is Arena and Houdini. It doesn't seem to be able to assign the correct hash all the time for Houdini engines. This goes for Houdini 1.5a, 2, 3, etc. This only applies to Arena as well. Chessbase doesn't have this problem, tested it myself. However, once you get Houdini to use the correct amount of hash, it should stay that way for a while, until you interrupt it. If you do it might or might not revert to 9mb of hash.
Now about the actual hash size: That depends on the engine. Houdini 3 has a formula for figuring out how much hash you need. Quoted from the user manual:
"If you know the average move time T (in seconds) and the average node speed of your hardware S (in kN/s), you can compute the optimal hash size with the formula: (T x S / 100) MB."
Therefore the faster your computer, the longer the time control, the more hash the engine needs to perform at its best. Note: You WILL get a small decrease in nodes per second but in the end your depth increases faster, and that is what brings home the deep analysis.
So if you're doing a 1 minute bullet tournament, you need almost no hash. If you're doing 15minutes, you might need 512, 1gb, or even 2gb depending on your computer speed. If you're doing a looooong analysis of one move, like you're setting Houdini to calculate a position overnight instead of running a tournament, you want the hash to be as much as you can set it.
One last thing to consider if you're using Houdini:
The minimum hash is 4. The max hash is 1,024 mb if you're 32bit, 4,096 mb for 64bit "Standard Houdini", and 262,144 mb on Houdini Pro.
The value is rounded down to a power of 2 (4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144 MB). In other words, setting 63mb of hash results in Houdini using 32mb of hash.
Hope that makes sense.