As a large lady myself, I have found this to be extremely difficult. Some patterns are easy to change to your size and some are almost impossible unless you are math whiz. If you find the sequence easy to follow then it is a whiz, but otherwise I don’t use the pattern. Ie, if you have a cast on that is 40, 44, 48, 52, 56, then logically, you would cast on 60 for the next size, but if the sequence doesn’t follow any specific then???
This site has some great patterns for purchase and I think the are quite lovely.
Check out http://www.knittingpatterncentral.com/directory.php and they have some stuff also.
As stated before, the lion brand site has patterns for larger ladies and gents too.