guess at (something)
To give a possibility of what something might be; to make a guess about something. We don't have to be exact here, so just guess at the measurements. Oh wow, I was only guessing at Cynthia's phone number, but I'm glad I got it right!
guess at something
to estimate something; to give an opinion about what something might be. I hate to just guess at it, but if you insist: ten feet long. Go ahead, guess at the number of pennies in this jar.