Which files do i edit to make the textarea for tag & cat description bigger?
Or maybe there's an easier way through functions.php to save having to do it every-time i update wp?
Hook onto the edit-tags page and insert some additional styling to adjust the textarea, the ID remains the same for post tags, categories and i think other taxonomies to..
Adjust CSS for taxonomy management page
Example only, tailor to suit you needs.
Hope that helps..