I wanted to surprise my sister with a top, too. But, after thinking about it, I knew that I wouldn't be able to guess at her size for a knit top, I'm not at all familiar with garment knitting, so I just told her I was going to knit a top for her & asked for a top that I could measure. I then told her that I wouldn't tell her when it was going to be done, that's as close to a surprise as we could get on that object :D But, I often do socks, which are fairly easy to size, especially if u know the recipient's shoe size. For other gifts, it's been hats, scarves, afghans, purses, things that I didn't have to measure.
Now, if u can sneak in your mom's house & measure one of her tops...that would be very nice, I think