This depends on whether you're building furniture for a general population or a specific individual. An individual can tune their furniture to their body.
If you don't know who will be using your furniture, you need to go for the average person to make your table fit the largest portion of the population possible. To that end, there are "approximate" standards, which are suggestions and an inch or two in either direction won't have a significant impact.
A typical dining room has a table at 30 inches and a floor to seat height of 18 inches for the chairs.
More generally (from a buying guide for furniture)
Regarding table and chair height:
Allow 12 inches between the seat of your chair and your tabletop.
Leave 7 inches between the chair arm and the bottom or apron of the table.
Regarding chair width:
Provide each guest 24-30 inches at the dining table.
For rectangular tables, add an extra 12 inches at each end for those seated at the head and foot of the table.