The mesh will probably be rusty, or soon will be if you expose it to the elements. Use a wire brush to remove any rust on the wire. Then give it a good coat of rust inhibiting paint. I like dark green paint. Painting by hand with a brush is time consuming, but if you use spray paint, 98% of the paint will be wasted.
These supports are plenty large for even vigorous indeterminate tomato plants. If you also grow determinate tomatoes you can cut a cage in half and get two.
These tomato cages are pretty time consuming to make. Figure about 1 ½ hours each, mostly brushing and painting. The good news is these are very rugged and should last a very long time. I made my first ones 5 seasons ago and they show little signs of wear. They might need new paint in a few years.