Follicles size. Know her fertile days?

My wife's menstrual cycle is 25 to 26 days. Last month Jan 25th was her due date but she got a late period on 29th Jan. But, this time bleeding was minimal and no cramps and other symptoms. After consulting doctor, she did a vaginal scanning this morning and said that there is a dominant follicle with a size of 1.4cm and asked her to come for a scan again on Wednesday. She also said that the follicle should grow to 1.8cm by Wednesday. Can we have an intercourse now or wait till the Wednesday scan to know the follicle growth? Is this growth is different from ovulation? I can't rely on online ovulation calculator as her cycle got disturbed this time and it is happening for the first time. So, how she would know her fertile days? Thank you