Length of Textbox which is from user input for different party

[solar]

Hi,

i need one info..

i would like to control length of text box based on another text box.

For example, Each Vendor having invoice number which may contain length upto
two or three character or may be more than that. In another form, When i
enter invoice number, the user should not enter more than specified
character. If they enter, it should not allow another text box.

For which, When i create Vendor name, i also get Invoice number's length in
the table itself. So, When i select Vendor name, and type actual Invoice
number for that vendor, the length of invoice number should not allow more
than the specified length.

I believe its clear.

So, How could i do this...?

Can anybody suggest me...?

Thnx..in advance.

ngr.