No, I'm not aware of any studies like that. It could be done. You could test people's satisfaction with democracy based on the size of the riding. I could look for those riding statistics, but I'm not familiar with them.
It's also not just a single question, of course. There is also this question of equity built into it for some citizens and for their representatives, as well. If you live in a rural riding in Canada that has just about the same number of people as an urban riding but which is huge, the quality of representation must suffer. Your MP is going to be less accessible. It's harder to get in touch with your MP. It's harder for the MP to get in touch with you. They have to travel around to learn about the needs and preferences of the constituents.
It's hard to say an exact number, but what we can say is that it will get worse as the constituency gets bigger or it gets more populous.