I did my application last month and when I did something wrong it would tell me and then I would do it right. You should be getting directions all the way through if you are doing something wrong.
I did the same visa, I got it for the whole 60days. Originally I was staying 90 days, however work commitments changed that and I am only staying 30. It is not expensive to get it for 60 days, however better to be safe when it comes to applying for the Thai pass. Maybe you may get rejected because not for your entire stay.