I just got my Speedy 35 a couple of weeks ago and I actually went through the exact same dilema as you, trying to decide between the 30 & 35! I decided on the 35 because I love bigger bags and I thought that besides being able to use the 35 as an everyday bag, it would also make a perfect travel hand-carry or overnight bag too!
It really isn't too big at all. I even tried on the 40 while I was at it and admit that the 40 was definetly too big as an everyday bag. With the 40, you would look like you're using your hand-carry. But I think the 35 is a great choice...I'm really happy I got it!
Here are the pics I took of myself carrying it and hope it helps you out a bit. I am 5'4, 106 pounds (size 2), for your reference: