If I understand correctly, you want to enlarge a medallion quilt. The original design has a center medallion 18.5" square and 6 pieced borders bringing the total size to 57" square. The problem is that unless we know exactly what designs the pieced borders use, what colors are dominant etc., we can't provide a very accurate estimate. However, mathematically we can get a swag:
Original quilt is 57X57 or 3249 square inches. The medallion is 342 square inches so the borders are about 2907 square inches.
Target quilt is 90X90 or 8100 square inches. Same medallion, so the borders are about 7758 square inches, which is about 2.66 times the original border area. You could take the original fabric requirements and multiply them by 2.66. Personally, I'd get 3X the original amounts. Which is why my scraps & stash piles look like they do!
The only way to get an accurate estimate is to figure out how many squares, HST, etc. for each block in each border, how many of those you can get out of a yard/meter of fabric, and how many you need for your new design...