The standard contemporary katana size is around 40 inches. However, the practical applications of the forms require a specific length or combination of lengths. When choosing a sword, you'll need to check for the measurements of three different parts: the handle or hilt (tsuka), blade (nagasa) and sword weight.
The length of the Tsuka (or handle) is very important so that we can have the proper leverage and balance. Depending on what training you are in the length required can vary.
The best way to measure yourself for a blade in my opinion is the tsuka-to-arm-length ratio. This method involves using the length of your forearm to get an idea of how long the tsuka should be. To do this you’ll just need a simple measuring tool. Get out a tape measure or ruler and measure the length of your forearm. So from your elbow to your wrist; this distance is how long the tsuka should also be.
Every sword style is different and you should always ask your instructor what length blade you should use. Traditionally, Short-bladed swords are ideal for close-quarter combat, whereas longer-bladed swords are preferred for combat in open areas. 
In terms of the most common height to nagasa ratio, please reference the table below. 
After length, weight is another important factor. Since a heavier sword is obviously more difficult to use, we suggest that beginners choose a slightly lighter Iaito to help them become accustomed to it. 
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