If you are considering to purchase an Iaito than some are aspects of great importance. This includes the right balance, ito wrap, tsuka shape, mounting quality and of course the length of your blade and tsuka (handle). This kind of thing will often be allright with a Japanese Iaito. You will not be able to concentrate very well with an Iaito that is loose in the Saya, has bad balance or is of poor quality. Already said many times but safety first, disassemble your Iaito, check it carefully and be assured that the Tsuka has two Mekugi.
To get your techniques, of Iaido waza, faster at a higher level, it is essential you have an Iaito with a correct length so you can perform the basic techniques and kata well. An Iaito with the correct proportion often already has a good balance so make sure that you can choose the right length, an option a good forge will offer.
The length is important so your techniques can be performed smoothly and decisively. Nukitsuke (unsheating) and Noto (returning the sword to the saya/sheath) are good techniques where you can immediately see if a length is correct. This will, of course, with a new Iaito, especially if you have trained before with a bokken, will take some time to get used to it.
A body and arm length are often in good proportion and with choosing an Iaito, you can hold the Iaito in one hand loosely to your body in a neutral standing position. The kissaki (point) is not allowed to touch the floor and should be +/- 5 cm with an 15 to 15 degrees angle above the ground. However this method is less reliable if your body and arm proportions are not average.
Fortunately there are schemes to determine the correct length for your Iaito. Your instructor may also may have a clear idea what is a good Iaito length good for you and may estimate body, arm and torso ratio well. The length is often measured in Japanese Shaku which 30.3 cm is. The tsuka or handle is measured in Sun (3.03 cm) and Bu (0.3 cm).
You should always ask first your instructor about the blade length because every style is different.
Here you can find an average Iaito and Body length scheme.