A few nice shots of those cars at roughly the same angle. Roughly. I've ordered these photos in a way that the floor width around driver and where side pods are mostly widest is the same for all cars.
Other than side pod width and (rough) estimation on relative wheelbase differences, we can also note the different approaches for front wing cascade turning vanes and Y250 zone.
Ferrari is a bit wider than Red Bull in side pod area, but also has a bit bigger undercut. More importantly, their actual inlet surface is about the same as Red Bull. Merc is wider with wider inlets as well, has about the same undercut width as RB, but also has less tapering in coke-bottle zone. Their 3 front wings are also very different (especially cascades and turning vanes, notably less aggressive on Merc), but have about the same aggressive Y250 curve on flaps. Overall - Ferrari and Red Bull seem to have some positive aspects over Merc.
Haas, McLaren and Toro Rosso have about the same side pod width, but Toro Rosso definitely seems sleekest to the rear. Haas has very Ferrari-like front wing (as was noted a few times), McLaren has their own approach with very mild Y250 flap curve. Toro Rosso is somewhere in the middle. Toro Rosso also seems shortest car out of those 3.
Massive side pods on Force India, as in previous season. They seem even bigger now, it has to be said. Their front wing seems to be the least aggressive on the grid, but it's still launch spec - so more to come. Renault seems the sleekest, but they have bigger inlet than Williams and smaller undercut.
Let's see how cars with same engines compare (sadly, it's Toro Rosso instead of Sauber).
Ferrari definitely sleekest in side pod area, but Toro Rosso is close by and all three cars have nice undercut and coke bottle taper. Seems Honda kept their size-zero philosophy even after McLaren departure.
All three Mercedes cars are very wide, but Williams has the biggest undercut on the grid. All three cars also seem to have widest hot air outlets, but it's very close between teams. Only Sauber stands out with biggest outlets of all teams. Interestingly, all three cars have the least aggressive front wing cascade turning vanes on the grid.
Renault cars definitely have least cooling requirements (if we can say that based on side pod width). McLaren is widest, but they had some trouble with overheating in pre-season. They have much to learn to be able to achieve Red Bull performance levels. Also, to close this little analysis up - Renault seems to have shortest wheelbase on the grid.
All photos taken from racefans.net, credits to XPB Images.