"Table cover" covers three quite different products, and picking the wrong one for the job is the most common mistake - a decorative lace-trim cover and a protective clear cover solve different problems, even though they sit on the same table.
If you want one cover that looks finished on its own, a lace-trim PVC/vinyl cover is the simplest choice - wipe-clean, decorative, and it doesn't need anything underneath it. If you already have (or want) a nicer cloth or embroidered cover on the table and just want to protect it from tea stains, water rings and scratches, a clear over-cover goes on top of that instead of replacing it. Cotton covers are the plainest option, best for a everyday dining or kitchen table where you want something breathable and machine-washable rather than wipe-clean.
Measure your tabletop rather than the cover you're replacing - a little overhang on all sides (2-4 inches) looks intentional; too much starts to look like it doesn't fit.
PVC and vinyl covers (lace-trim or clear) just need a wipe with a damp cloth - never machine wash these, the coating can crack. Cotton covers are machine washable in cold water like any other cotton furnishing.