The content of each cell in the Spreadsheet is kept into a div element, so adding MaxLength attribute is not an option. A possible approach you can use, though, is to attach a handler to the change
event of the control and check manually the length of the new value, cancelling the event should it exceed the target max length.
For example: http://dojo.telerik.com/imIBaTIn
You can also consider using a range validator.
You can see a sample where such is used here: