How do I properly get my feet measured? Podiatrist? Running shoe store? My Brannock size is 2.5 sizes above what I currently wear and my left foot is 1 size larger than my right foot. Even when I try to go up half a size, I get terrible heel slip.
Even between some shoe brands, some will run big, others run small, so finding what works best in terms of specific shoes, not just one particular brand, is going to be the best thing to do, so either spend a while trying on a lot of different shoes in any place that has a good range, including whatever size you might usually go for, a size down or even a size up, just to see.
If you are in USA or other places that have half sizes you might be more in luck with finding something that works better for you.
Here we only have whole sizes, so getting something that fit exactly was a pain, but one thing I did learn to do with shoes that were just a bit snug was get a shoe stretcher and stretch them out, as well as widen the toe area for a specific ingrown toe nail issue on one foot.
Using a shoe stretcher on whatever shoe for your larger foot might work better than trying to get shoes that are either going to be too big on one foot just to fit the other, or struggling with other issues.