I just discovered the following:
When I create a user field in the UI or DI with a size of x The system allows me to enter valid values with a size bigger than x. This happens in the UI AND DI, no error or warning is issued.
When I open my form and choose the large valid value, not the description but the valid value, truncated at x characters, is shown.
There is no check whether the valid value is larger than the size of the field.
I have tested it in SBO 2005 SP01 PL31