internal storage - Does a new software update take up more space, or replace the space of the old version?

Just today my Galaxy s6 received the Android 7.0 update. I haven't installed it yet, though, because I'm concerned about space. Unfortunately the Galaxy s6 doesn't have external storage so I have to be very careful about managing my device. I have two questions:

1) How big is Android 7.0 for my phone?

2) Will Android 7.0 take up that amount in #1 PLUS what Android 6 takes up already, or will it replace what Android 6 takes up and hopefully leave me with very little decrease in storage?

Thank you!