If they're 14s, it'll be a ST205/75R14. Only those two sizes were used in '99 on non-slide trailers.
To get tire covers, they usually go by tire diameter and width. You can look up your tire brand and size on tirerack.com to get those specs, or just look at the chart on Adco tire cover packaging to find the size you need.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR