Thank you for reaching out to Constant Contact API Support.
Currently the maximum length of custom fields when using our API is 50 characters. While you can have up to 255 characters in the website UI, our current V2 API was built for our previous contacts system which only supported 50 characters, and this is a holdover limitation from that. We are working on a new V3 API version which does support custom fields up to 255 characters. Unfortunately I don't have a specific timeframe for you on when that will be available.
Please let me know if you have any other questions!