Hi. You can put any blade length you want in to an US saber as long as the diameter is 1" and the saber is not a thin neck as they use a specially prepared tube section that seats in the emitter. However, there are other factors to consider but the main ones for your daughter would likely be her height, length of saber and what she would be using the saber for. For example, a v2 Initiate stunt saber is too short and light for a 36" but possibly would be balanced okay with a 32" midgrade blade. The other Initiate lines and the Apprentice are more hearty sabers and are fine with both longer and heavier blades and the shorter blades including the 16" ones. So all lengths covered lol.
The 24" would be a good starting blade length and there could be up to 2" or so lost inside the emitter to keep the blade more secure. Many adults have use for this shorter blade as well.
In the future please be a bit more specific regarding the type of question for your subject title.
Here a photo of blade lengths 16, 24, 32, 36 inches.
