When you're looking along the timeline in the sequence, have you tried changing a couple of the options?

Click on the little box under the Eye icon to the left of the Video Timeline (the option is called Set Display Style) and select Show Frames.

Next, move your pointer to the line that divides Video tracks 1 and 2 at a point where the pointer changes to two horizontal lines with an arrow up and down, now drag the line up and you should see the thumbnails display much bigger.

As far as I can tell, the only limit to how big the thumbnails can be is the size of your screen, however, do bear in mind that because of the size of the frames thumbnail, they will cover a number of frames and therefore scene change often occur prior to their thumbnail, but at least it gives you a better indication of where breaks are.