A maximum length for a float value doesn't make sense.
If you want the user to enter any float value representable by java you want allow thing like
1000000000000000000000000000000000000000000000000000000000000000
or even
000000000000000000000000000000000000000000000000000000000.00000000000000000000001
Limits for input fields should be based on business needs not on rules like "I have a limit on all other fields".
The "business" rule here so far seems to be "Can be parsed and stored into a float"
Also note that limiting the input length often prevents input (via cut&paste) of stuff that is longer and only becomes valid input after some editing. So it actually reduces usability.