Welcome to the MYOB Community Forum, I hope you find it a wealth of knowledge
I see you have posted on the MYOB Essentials Board, in terms of MYOB Essentials the Notes section on the Contact window allows for 2,000 characters.
You may be referring to the AccountRight range in which the Notes field on the Card Detials tab on a card is limited to 255 characters. Currently this is a set character limit and is not able to be changed. However if you would like to see that field expanded I would recommend showing your support for Increase size of card notes field on the AccountRight Idea Exchange.