Thanks for moving the thread.
You're going to need to explain the datatype of the fields you're importing (show us your actual Marketo field config). CSVs do not place any limit on the length of quoted string fields.
In fact, CSVs don't place limits on the length of unquoted strings, either. When unquoted strings are interpreted as numbers, there can appear to be a limit on characters. But that's misleading. The limits is actually on the size of an Integer, not the "length" in digits of a number-like string. For example, a 32-bit signed integer can have a max value of 2147483647 (just over 2 billion). That happens to be 13 characters long when printed as a decimal string without commas, but the underlying limit is not related to the number of printed digits.
You may be using an app that truncates numbers-as-numbers. If so, set column type to string/text instead. Again, CSVs themselves have no such limitations. They are just text files.