I really want t get some full chaps but I'm wondering if it's possible to make modifications?

This is definatly a possiblity! They're may also be shops around town to gte them hemmed up or even properly fitted. I do not think if you bought these chaps they would get loose, this type of material does not usually stretch. I think you'd be safe. I've had half chaps for almost 3 years now and full length chaps for 3 years as well and they have never stretched out shrunken. Hope this helped :)