Bigger root in default installation?

First post here.

Welcome!

I´m slowly moving to the world of open source (school stuff doesn´t really work with this world).

Trisquel actually supports free software: https://www.gnu.org/philosophy/open-source-misses-the-point.html

Here are arguments to convince your professors to switch to free software: https://www.gnu.org/education/edu-why.html

I´ve somehow managed to get root too full.

A few easy commands that may save you some space:
$ sudo apt autoremove
$ sudo apt clean

You can also analyze what takes space on you root partition (for instance using baobab, which is in Trisquel's repository) and maybe decide to remove some space-consuming video games, old logs, ...

I guess 20 Gb is not enough with my stuff.

20 GB is indeed not that much nowadays. I think it would be desirable to at least double that default size.

How can I set the size during installation?

You can choose the "Something else" type of installation (if it is still called that way) that will let you define the partitions the way you want. By default, beside a UEFI partition (necessary on most computers nowadays), Trisquel's installer setups an ext4 filesystem mounted at / (the root of the file hierarchy, hence a necessary filesystem), an XFS filesystem mounted at /home and a swap partition. Well, if it has not recently changed (I have not tried a Trisquel 10 ISO yet). It is usually a good partition scheme, but it cannot suit all use cases.

In Debian installation this is much more simpler I guess.

I would say Debian's installer is actually (slightly) more complicated than Trisquel's.

After installation I´m not able to resize the root because there´s swap and home in the way.

XFS (Trisquel's default for /home) cannot be shrunk. That complicates the operation. It is possible anyway.

Should I just try to fix this during install again (and again) or something else?

To avoid reinstalling everything, you can backup /home on another disk (maybe using Back In Time, by default in Trisquel 9) and, from a live system, maybe using GParted (by default on Trisquel's ISOs), you can delete the related partition (and maybe the swap partition), enlarge the root partition, recreate a partition for /home (and maybe a swap partition) in the whole remaining (but now smaller) free space and edit the /etc/fstab on the root partition of the installed system to properly identify the new partition(s). For the last step, there is no need for UUIDs: you can write /dev/sdaX in /etc/fstab's first field, substituting X with the proper integer(s). You should then be able to reboot on the modified system where you can recover the backup.

Well, that is one option at least. You could also try to copy partition for /home on another disk (GParted should be able to do that) and copy it back smaller. Or you could decide to only have a root partition taking almost the whole disk (everything but the space occupied by the UEFI and the swap partition). Or...

Just a quick update to this (quite late post as I've been busy).

Now I'm using X200T with Libreboot and Trisquel 10, which is working very nicely.

The problem with root being too full. I made a completely new installation to my libre Thinkpad and got it working with encryption and custom-sized root partition. So everything is fine here (well, almost).

So, thanks Magic Banana for tips!