The NES games of the Nintendo Switch Online service will have an estimated size of up to 58MB, at least in its initial download. We can assume that the main reason for this size lies in the menus and the additional systems integrated in the games, for example, integrate online services and save the games at any time.
The Nintendo website indicates that the download size of the application that gives access to the games is 58.72MB in the section of Nintendo Entertainment System - Nintendo Switch Online.
Depending on how the games have been produced, it will be necessary to see how much space the service occupies in the memory of the users' console.
We remind you that the games are downloaded to the local memory of the console but, in order to remain active, the player's Nintendo Switch must connect to the internet at least once a week.