Change field size for email notification

[DFC99]

Hi, I'm running a forum (v. 3.6.8) which contains a private board, where some of the members would like to receive e-mail notification of new posts and new threads.
However, the field in

Admin CP -> Forums & Moderators -> Forum Manager -> Edit Forum -> Email Addresses to Notify When there is a New Post

is limited to 250 characters.

Where and how do I manually change this to, say, 2000 characters?

I'd be really grateful if somebody knows the answer ).

(p.s. a step-by-step explanation in layman's language would be great!)