Thin lining

Hi so I have done 4 rounds of clomid decided to switch to letrozole at 7.5 mg I went in for my cd 17 ultrasound to confirm ovulation and my lining was at like a 6 mm maybe he said he is not sure if I ovulated since it’s a thin lining. And the follicle doesn’t look over 2cm but after ovulation doesn’t the follicle shrink some? And is 6 mm to thin for 1dpo ?