We are wanting to create a custom field in marketo to contain a promotion code a contact has redeemed. Is anyone aware of any character limitations in custom fields?
You should be fine. You shouldn't come anywhere close to reaching the character limit.
Most fields don’t max out at number of characters, but instead at amount of bytes. Because of that, we’re unable to provide a definitive character limit for each field. The two exceptions however are Text Area (3200 characters) and String (255 characters).
Custom Field Type Glossary - Marketo Docs - Product Docs
Please move the thread to Products and someone will answer it there. "Community" is for website feedback, not product support -- and yes, this is confusing!
Just a note that Text (textarea/longtext) is 32000, not 3200. And in some input contexts it's 30K instead of 32K.
Also, the docs are kind of strange on the point of bytes vs. characters. Indeed that distinction makes sense when you're talking about string fields (variable-length encoding). But it doesn't make sense when it comes to, for example, integer fields. There is a finite number of characters in any 32-bit int field when it's output as a string, even ifthose aren't actually "characters" when the field is stored. That is, an signed int can hold a max value of 2,147,483,647: that's "2147483647" or 10 characters. You can't store "9999999999" but you definitely can't store any 11-character numeric string.
Retrieving data ...