The minimum room size required in practice is 120 feet wide and approximately 15 feet high
If you want to throw Lightning call Inside, we have to find out how big the room should be to accommodate the cloud. Here is the relevant spell text from the last errata:
A cylinder-shaped storm cloud appears that is 10 feet high with a radius of 60 feet, centered at a point that you can see within the range directly above you. The spell fails if you cannot see a point in the air where the storm cloud might appear (for example, if you are in a room that cannot accommodate the cloud).
So, for the cloud to fit inside, the room must be 120 feet wide and 10 feet high. However, the cloud must be "above you," and since it is approximately 5 feet tall, the total height must be approximately 15 feet so that a 10-foot cloud fits over you.one Keep in mind that the room must be 120 feet wide everybody addresses. A narrow corridor 120 feet long will not be enough. Also note that if the room is exactly 120 feet wide when should Stand in the exact center of the room to cast the spell successfully, since the center of the cloud forms "directly above you," and the cloud must be exactly centered in the room to fit.
In more general terms, you can only throw Lightning call indoors if it is at least 10 feet high above you and at least 60 feet between you and the nearest wall. Obviously, you will rarely find a room large enough to cast the spell, which makes it an outdoor spell only about 99% of the time.
oneTechnically, you can cast the spell in a room only 10 feet high, in which case the cloud will extend from the ceiling to the floor. However, in this case, you may have difficulty pointing to your rays, since you can only point to one point you can see. The spell does not say whether the cloud obscures the vision or not, but the DM might well decide that the cloud works similarly to a Cloud of fog spells and causes heavy darkening in your area. If you do not want to leave this to the interpretation of the DM, you must leave enough space for yourself under the cloud.