There is no reason to communicate over the network when the player selects units, because in most games just selecting a unit has no game-mechanical consequences. So this is an information which isn't relevant to the server or to the other players.
But what would be important is when the player gives a command to one or more units. When issuing a move-command to a bunch of units, the client would send the IDs of the units and the locations to which they were sent. When we assume that a unit has a 2byte ID (allowing up to 65000* units in the game), the target coordinates are in tiles and the maximum width and height of a map is less than 65000*, we have 6 bytes per unit move command. When the player sends a move-command for 200 units at once, we have 1200 bytes.
The best RTS players perform up to 200 actions per minute, which would be about 3 actions per second. When each of that actions is a command to 200 units, this would require a bandwidth of 3600 bytes/s. Let's round this up to 4 kByte/s with overhead.
4 kByte/s is half the bandwidth of a 56k analog modem which was a standard consumer-grade internet connection in the 90s when Starcraft 1 was released. Assuming that the incoming bandwidth is about the same, we see that a duel between two world-class Starcraft players was just within the bandwidth capabilities of the average household when Starcraft 1 was released. Considering that not every action will be a command to 200 units at once and that the average player performs much less than 200 ApM, we see that an 8 player online match between casual players was definitely possible that way.
*65536 when you want to split hairs