A typical dining room chair is about 18 inches high at the seat. That means you have a 12-inch space between the standard 30-inch dining table and seat, to accommodate your lap, napkin, formal or informal attire and any random bits of food that go astray. So, figure chairs with an 18-inch height as a starting point. But always measure your particular dining room furniture to find the exact right height for a comfortable and compatible relationship.
Not all dining tables are created alike. Yours may range from 28 to 31 inches tall, and you'll have to adjust chair height so your guests don't wind up with their chins on the table or perched in midair. Chairs could be between 17 and 19 inches from the floor. Antique and handmade dining tables may vary more widely from the standard measurement and call for inventive seating solutions. Set a rule to leave between 10 and 13 inches from the chair seat to the edge of the table, and tweak that to match your circumstances. Wide girth or taller people will be more comfortable with more space, so look for chairs that provide extra inches between seat and table. If your chairs have arms, they should fall about 7 inches below the table apron in order to slide in and out easily without any arm-bashing.
You've fallen in love with a hand-carved elm Ming table that will star in your dining room. It also happens to be 33 inches tall. It's going to be tough to find dining chairs to accommodate that height, but there are two choices that will work fairly painlessly. Buy or commission stools of the ideal height, to use in place of chairs. Or buy standard chairs and add very firm cushions to boost diners to an appropriate height at the table. For any odd-sized dining table, keep a tape measure handy for the times when you spot the perfect chair or chairs. Some antique, custom-made and rustic chairs will have quirky measurements that are made-to-order for your treasured and nonstandard dining table.