As Skip said, the help section gives you some hints as to how to redim arrays. Note that when redimming you can only change the number of elements in the last array if you want to preserve the contents of the array. i.e. if you have your 100x100 array partially full of data and want to trim it down but keep the contents, you will only be able to redim the array to something like 100x50 or whatever. (not 50x100).
I use a particular method to get around this problem. If you want any further details dont hesitate to ask.
Matt