Size of Text Fields in "Custom Fields" is 250?
Is possible to made them bigger?
Unfortunately no, it's not possible at this time to make them bigger, sorry, you'll have to work within the 250 character limit.
Thanks a lot of!
Will longer text just be cut off, or is there a warning message?
I use a text field to individualize my holiday mailings. 250 characters is fine for that but it would be a desaster if the text wouldn't be complete, just because I didn't count chars correctly...
Each custom field value has a data limit of 250 characters which means that you will not be able to add any more characters past 250. If you're worried about incorrect calculations, you could use a website like this one or the word (and character) count feature in a word processor.