The minimum required room size in practice is 120 feet across and about 15 feet tall
If you want to cast Call Lightning indoors, we need to figure out how big the room needs to be to accommodate the cloud. Here is the relevant spell text as of the latest erratum:
A storm cloud appears in the shape of a cylinder that is 10 feet tall with a 60-foot radius, centered on a point you can see within range directly above you. The spell fails if you can't see a point in the air where the storm cloud could appear (for example, if you are in a room that can't accommodate the cloud).
So, in order to fit the cloud inside, the room needs to be 120 feet across and 10 feet tall. However, the cloud needs to be "above you", and since you are about 5 feet tall, the total height needs to be about 15 feet in order to fit 10 feet worth of cloud above you.1 Note that the room must be 120 feet wide in all directions. A narrow hallway 120 feet long will not suffice. Additionally note that if the room is exactly 120 feet wide, when you must stand in the exact center of the room in order to successfully cast the spell, since the center of the cloud forms "directly above you", and the cloud must be exactly centered in the room in order to fit.
In more general terms, you can only cast Call Lightning indoors if you have at least 10 feet of headroom above you and at least 60 feet between you and the nearest wall. Obviously, you are very rarely going to encounter a sufficiently large room to cast the spell, making it an outdoor-only spell about 99% of the time.
1Technically you can cast the spell in a room that is only 10 feet tall, in which case the cloud will extend all the way from the ceiling to the floor. However, in this case you may have difficulty targeting your lightning strikes, since you can only target a point you can see. The spell doesn't say whether the cloud obscures vision or not, but the DM could well decide that the cloud works similarly to a Fog Cloud spell and causes heavy obscurement within its area. If you don't want to leave this up to the DM's interpretation, you'll need to leave enough space for yourself under the cloud.