How to make small logo progressively get bigger?

I am using a logo in .png form. I would like to have the logo very small at first and then get bigger frame by frame as it goes down the timeline. I created a keyframe on frame 1 and then another keyframe on frame 40. I then made the logo on frame 1 a small size, and I have the logo on frame 40 a large size. I am not sure how to tween them to produce the effect. It won’t let me select the “shape” tween. I tried “classic” tween and motion tween (makes you convert to symbols), but no dice.

Thanks,
Kris.