Is there anywhere online that I can see what different solitaire carat weights look like on actual hands? I am still debating the 1/3 ct Tiffany & Co. or the 1/2 ct Blue Nile/James Allen promise ring purchase.
Yep - here. There are lots of threads that are treasure-troves for planning: try using the search function with phrases starting with "Show me your _____"; there are threads on everything from finger sizes ("Show me your size 5s!") to the ever-popular "Under One Carat Diamond" thread.* Not to mention the more esoteric ones, like the ones on rose gold and baguette halos and fishtail prongs and everything else under the sun ....
*I may be a little off on the exact titles of these, but the phrases should bring them up.