As a guide, dining room tables are usually 76cm high while the standard seat height of dining room chairs is approximately 46cm. This distance between the seat and table surface makes it comfortable for most people to sit.
Note whether the chair seat is hard or upholstered. Upholstered seats tend to compress when you sit. If the padding is thick, the compression may be substantial.
You'll also need chairs that actually fit under your table. If they don't, your diners won't feel comfortable and you'll damage both table and chairs.
Make sure there's room to slide the chairs underneath without bumping each other or the table's base or legs. If the chair seats touch, diners feel cramped and uncomfortably close.
If you use dining chairs with arms at any type of table, make sure the tops of the arms don't brush or bump the bottom of the tabletop or apron. In addition to the inevitable damage your chair arms will suffer, diners may not be able to sit close enough to the table to eat comfortably.
The final scale concern when choosing chairs for a mixing room table is the difference between the table height and the overall chair height. Make sure the backs of your chairs are taller than the top of the table. Taller is better, but a height difference of two inches is the absolute minimum. The chairs look squatty otherwise.
In addition to choosing tables and chairs of compatible scale, the pieces need to look good together.
Choosing tables and chairs with a common element usually ensures that they'll look good. That common element can be the period, the colour undertone of the finish or the level of formality. It can even be a single design element, such as the furniture legs or feet.
As with most decorating rules, there are exceptions. When mixing a dining table and chairs, the exception is when the pairing works because it's so outrageous.
You'll still get raised eyebrows from some of your pals, but the fashion-forward folks on your guest list will wish they'd thought of it first.