I want to build a sewing table using a counter top.

https://www.hunker.com/13404245/the-average-height-of-a-sewing-table

I would go to a store that sells these and sit and see what would be comfortable. Then measure that sewing table. It may depend on height and what type of chair.

It depends on your height. This article gives you some things to consider:

https://www.craftsy.com/blog/2013/09/sewing-tables/

Put adjustable legs on the counter top. I use both a sewing machine and a serger. One is a bit taller than the other, with adjustable legs I can get it to fit me or my mom. Having it at a comfortable ht takes stress off certain joints.

Sit in the chair you want to use and compare that to your kitchen table. Is it too high to sew comfortably? Too low? I'm tall, and I like room under the table so I can sit and move around. I had my kitchen counters installed higher than average and it saves my back! You could also start with the height measurements of a craft table you like online or in person. If you are building it, make it work for you. Ana White's site has lots of free plans to help you get started.