It's as big as fantasy land. Using the space to gain some real capacity (assuming they actually build 4-5 attractions total) is a win.
Disney frequently mismanages and wastes space with their expansions
New Fantasyland was advertised as "the largest expansion in Magic Kingdom history" and claimed real estate from Toontown Fair, but Fantasyland today has the same number of rides as it did in the 70s.
Tron was a true expansion to Tomorrowland's footprint, but the losses of Stitch's Great Escape, the Galaxy Palace Theater and the Skyway have yet to be replaced
The Disney of Today needs 12-14 acres for two rides. Even for something like Toy Story Land.
The Rivers of America were built to support 4 attractions. Replacing it with 4 other ones is not the capacity gain people are trying to make it, especially when other attraction, retail and dining space in the park remains abandoned
It would be a lot simpler, for example, to reopen Tomorrowland Terrace than build a new cafeteria elsewhere in the park.
People supporting this project because they personally don't like the RoA doesn't mean it's the best use of park space or money. Reopening dead space and other proposed projects like Moana in Adventureland and the MS Theater would be.