Take a look at:
They both have comparative charts to tell you what length of sword to buy when you are looking for an Iaito - and the same rules aply when you want a bokken.
This is a part of a long text about blades on: "If you are of typical non-Asian height (5'10"-6'), look for a 26" to 29" blade length. Hold the handle with right hand at front next to the tsuba, and carefully let the blade hang down at your side, arm relaxed. Don't let the tip hit the floor or you've just bought a blade with a bent or broken tip! A correct length blade should come close to, but not touch the floor."
You can find the whole thing on:
Apart from the right length it should also have a good 'fell' and a curvature of the blade that suits you. I prefer my bokken which is slightly more curved than usual, but it takes time and experience to get to know what you like. I like as well to have a bokken which is not to light and with a balance closer to the middle of the bokken than to the handle. It's very much a question of personal likes and dislikes, so the best thing for you to do would be to go somewhere with a large selection to try out.
Finally it must of course be absolutely straight when viewed from above. Even a slight curve to one of the sides could introduce very bad habits into your way of cutting. Try placing it along a line you know is straight and check it out.
Hope this helps!