I hope im not puttin in my mouth but the answer is NO. Atleast not with this version of the board. The next version may support small words but this one doesnt.
Its a limitation of the board software and has to do with how the database stores the search words or something to that effect. I think it is to elimitate a masive database of useless search words such as "if to do" But if i recall correctly from when i was messing with message boards this limitation has been solved in many board systems. So the next board due out in a few weeks i think may support less letters.
The link below is straight from Invision Boards Demo. If you attempt to search a 2 letter word there you also get a error. So it looks like this is still going to be a problem with the next board also. We should all take this into consideration when posting topics. Make sure you spell out acronyms atleast once in a post. This way it will be easy to search for. If all else fails then just post your question and maybe one of us will recall a thread and point you to it.
It is possible, if I change the way it searches messages. The other searching method seems to be slower, so I have to do some testing, but it won't happen until we switch to the new upgrade (should be soon tho).