Considering buses, I always use two or three tiles for each stop (more for hubs of course). If I make smaller stops, they get overcrowded even with the highest-capacity stop. This size is a good compromise between capacity and covered area (and thus distance between stops). For the same reason, my subway stations are not longer than 3 tiles. 2 or 3 tiles is also a good length for trams.
About trains, when you start a game, you would better make short stations and use short convoys as station maintenance costs are high. Of course, start with the lowest capacity stations, and when passengers level increase, make the station longer until you reach desired length for the long term. Then, if passengers level keeps increasing, upgrade your platforms with higher capacity ones. It's hard to keep playing with small convoys all the game, the amount of passengers gets too high in some areas, so you have to make much more convoys than if you had long convoys and long stations, and your network quickly saturates.
A saturated network is a big trouble for two reasons: it can jam anytime and convoys are much slower as they have to stop often behind the previous convoy (therefore you loose much money since you can't get speed bonus, and, as passengers flow is slower, your network gets even more overcrowded than if you had less convoys).
In my games I often use 5 or 6 tiles long stations for usual train lines (less for very light lines), and 7 or 8 tiles long stations for high speed lines (high speed trains take less passengers and needs more space between convoys to keep speed high, so having long convoys is a better solution).
Anyway, after some years of game, having long stations is not so costly because you can use very low capacity platforms (lowest is 32 passengers per platform tile).
EDIT: For goods: as freight convoys don't need to be so fast, the best solution is to have long and heavy convoys to make most profit from the loc. Anyway, in 1930 locs are so weak that 6 tiles is already very long even for freight trains.
EDIT²: By the way, keep in mind that price and running cost of a locomotive depends much on its limit speed (because of speed bonus which is very high for passengers), so if you make freight trains, use a slow, but powerful, locomotive.