Access fields with the data type set to Text are limited to 255 characters [or the number specified in Field Size, whichever is smaller]. Try to insert longer strings into a text field, and you'll either get an error message or have the data cut off at the 255-character point.
There is an easy [and easily-overlooked] way around this limitation, though: set your field's data type to Memo. Memo fields are much more generous, allowing you up to 65,535 alphanumeric characters.
Also, according to Access Help, "If the Memo field is manipulated through DAO and only text and numbers [not binary data] will be stored in it, then the size of the Memo field is limited by the size of the database." Simply, with ingenuity you can exceed the 65,535-character limit as well.
For any alphanumeric data that might exceed 255 characters in length, don't employ the Text field type. Go with Memo.