First time I bought a bar table online, I ignored the height. Showed up, put it next to my counter, and it hit me at chin level. I measured the counter, not the bar chairs I already owned. That's the mistake. Bar top tables are basically a different species from standard dining tables.
The only number that matters is seat-to-table clearance. Not the table's height alone. Not the stool's height alone. The difference between them. You want about 10–12 inches between the seat and the underside of the table. Over that, you're reaching. Under that, you're hunched.
Most bar tables run 40–42 inches tall. Most bar stools run 29–31 inches. Do the math. If your stool is 30 and your table is 42, that's 12 inches—fine. But if your table is 40 and your stool is 31, that's 9 inches. Your knees will remind you every night.
So before you click buy, measure your existing stools. Write the height down. Bring it to the product page. Compare. If the table lists a 42-inch height and your stools are 29, you're golden. If you're buying stools separately, buy them together so you know they match.
Also, check the bar top table's base. Sounds boring, but a center column with a heavy base works better than four legs for actually sitting down. Four legs mean the outer legs get in your way—you can't slide a stool under the table the same way, and your shins pay the price. Center column, you can tuck in and there's actually legroom.
One last thing: the overhang. If the table has a foot rail or a lower shelf for feet, that's great, but make sure there's enough depth that your legs don't hit the shelf. I've seen tables where the foot rail sits too far back, so you either rest your feet on it or you're dead-legged.
My rule now: any bar top table I even look at, I check the seat-to-table clearance first. Everything else—material, finish, style—is secondary. If that number's off, the table's useless.
So measure your stools. It's the one factor that decides whether you love the thing or just live with it.
More on the topic: A Plain-English Look at High Top Dining Table Set For 4 and Round Coffee Tables for Sale: 7 Things Before You Click Buy.