That's a bit of an odd place to be getting the error if you're running a stock 7.5.3 install, since that is just where it sets up any custom bbcodes you might have for the markup panel. So that appears to be coming from the new post screen, which really shouldn't be taking that much memory.
Are you personally getting that error? If so, what were you trying to do at the time?
If you really need to up the memory, then you'll find this in your server php.ini file, and it's the: memory_limit setting you're looking for.
