If I choose "something else" for installing Ubuntu 20.04, in which partition should I install it and how large should be this partition?

First, let me make clear that I have gone through the answers to similar questions and still have decided to post my own because: i) no answer seemed specific enough for a relative novice like me, and ii) in any case, an answer updated to the size of Ubuntu 20.04 would likely be helpful to others as well.

Now to elaborate a little on my title. As mentioned there, I have chosen the "something else" option for the installation. The 'swap' and 'home' partitions present no difficulty but exactly what partition should I create/choose for installing Ubuntu 20.04? And what should be the size of this partition so as to comfortably accommodate not only the OS now but also the future updates,etc? Is it the 'root' partition and does the installation automatically choose it? In that case the 'root' partition has to be allocated a suitable size.

Secondly, a main concern of mine is to keep the additional software installed by me as well as my documents (text, images, videos, etc.) untouched in the event of a re-installation of the OS. So I am concerned to know where exactly this kind of data are to be stored. In the 'home' partition, or in a separate partition created out of the remaining free space? If it's the 'home' I will have to make it sufficiently large, and if it's a separate partition/partitions I would like to know its/their mounting point(s).

I shall be grateful for all precise, clear-cut advice.

The simple answer is 25GB for /, 1.01 × RAM for swap, and the rest for /home. System updates will only affect /, and if you later do a reinstall, you can tell the installer not to format /home.

Follow this guide for more details : https://www.tecmint.com/install-ubuntu-alongside-with-windows

If you choose something else:-

note:- 1. all your software are gonna store in root directory

2. all your pictures, downloads and movies and other stuff you download you can save it on home partition

3. Swap partition is basically for support your system's ram

Now make sure to allot more than 20Gb in root partition because every software are gonna store there

note:- steam games can be saved in the home directory

And that's it.

Installation steps

1. create root partition with / sign with 20+gb
2. create a home partition with /home written
3. create swap partition according to you system ram

note:- make sure to make all these partitions in logical type not in primary

Honestly, I don't see anything in your proposed usage that requires "Something Else". Your proposed use looks very default-settings. Everybody wants their data and locally-installed software protected. Everybody wants some swap.

Advice: Make your first install successful rather than perfect. A success --sticking to the default installer settings-- takes only about 45 minutes or so.

After you know what success looks like, you can re-install with more complex settings. Another 45 minutes.

• Protecting your data and locally-installed software is the job of regular backups. A separate /home can be overwritten by an admin mistake or simple typo during a re-install. We all think we won't make that typo...but we do.

• The Ubuntu installer creates a swap file nowadays instead of a swap partition. You can indeed specify a separate swap partition if you wish, but most folks have no need for that customization. The file is easier to grow/shrink if your swap needs are wildly different than expected. The partition is needed for hibernation.