SRT files don’t have any built-in functionality to position the text. There’s other text-based subtitle formats that provide such features.
A lot of things are technically possible… for now at least there’s no solution as you’re describing it within Plex (beyond the top/middle/bottom positioning of text-based subtitles). Keep in mind that changing the positing of image based subtitles is even more complicated as those can really place the text pretty much all over the screen.
There’s some related feature requests… (seems the people discussing it haven’t actually voted for it yet… you can be the first 
):
