Trying to figure out which size base to use can be a problem sometimes. It can be fun to go above and beyond with your modeling and make something really unique but you don't want to go overboard either.
Don't get me wrong, there are lots of great 25mm bases out there but with the larger sizes, you can express yourself a little more.
With Sarpedon, the trick was getting just the right size base so that he looked like he "fit" on there and neither portion was too big or too small... just right was what my client was looking for.
The top picture is a mock up on (over) a 40mm base... way to small. In the end, my client chose the 60mm size base that allows just enough room for the legs to fit without being cramped but not so big that it starts to affect game play.
As luck would have it, Game Vault was increasing their selection of 40k bits and I was able to pick up a 60mm resin "urban" base from Dark Art Miniatures that fit the bill perfectly.
My Project Link: Follow my Sarpedon Conversion