As indicated by the Alan Shaw quote, the main factor affecting the performance of a speaker cable is the aggregate area of the conductors…to a point - as discussed by @raysmtb1. IOW, if you have enough, then more will probably not change anything. How big you need is affected by the current you are driving, based on the efficiency of your speakers and the output of your amplifiers.
However, the other cable materials and construction factors can also make a sonic difference. As an example, my go-to choices here are both bi-wired cables, a stranded copper cable with two runs of 7awg to each speaker and the other being two 9 awg runs of a multi-strand, individually insulated, solid core copper cable. I can tell the difference between them but I like them both and probably 4 times out of 5, I prefer the smaller, multi-strand, solid core cable.
if you are pushing a lot of watts, 15awg could be small’ish. However, as others pointed out, you can go larger without breaking the bank.