I have a gaming laptop with a 125gb ssd and 1 TB secondary drive. Ssd has just 20 GB free.

I wanna dual boot windows 10 and arch. Most of the secondary drive is full of steam games. Win 10 for gaming and arch is for projects.

What's a good way to partition the drives for arch? I can't guess how much space either will need in the future. And what do I do with the other drive?

I got 20 gigs on my ssd for arch and the rest for Windows. For data storage I use one of my other drives. Arch can read/write ntfs so you can use a folder on there for home and the rest for steam.

But it really depends on what you doing there. I have vscode, git and the rust ecosystem on there. And I have plenty of free space on the 20gbs I gave arch

For your own convenience and to prevent another "windows killed my linux partition" rant, put them on separate drives. At least separate the boot partitions. If you go with both systems on SSD, put the arch boot partition / uefi partition on the HDD. Set the HDD first in boot order (or higher then SSD) and add a boot option pointing to windows on the other partition and one for arch.
That way windows can do with it's boot mechanism whatever it wants and it cant break your linux boot stuff, because it's not there

@Kimmax will there be any speed difference between running arch on an ssd vs hdd?

@hashris like night and day

@hashris of course. It'll likely add a second to your boot time if you put the boot partition on the HDD

Buy another ssd and share the 1TB hard disk