I'm planning on eventually buying an ssd for my laptop. I run Ubuntu and mostly just download a few pictures, and occasionally a Linux distro for other people. What is the smallest ssd I can use and not feel like I'm running out of space? 32gb, 64gb, 128gb?
Go for either 64Gb, or 128gb. Or you may wait, since the SSD market is still pretty new in the private hardware world; they're gonna get a lot cheaper in the future. You can get some pretty decent 64/128Gb SSD's for a reasonable price though.
For OS system's I would go for a 64GB or a 120GB, The best SSD I have ever had in a price range is the Samsung 840 Series it is around £80-90 I highly recommend it if you do not want to spend a lot of money. I hope this helps.
cromebooks also don't store anything locally. If you will always be on a network, and dont want to have any linux games or anything, that would be a good option.
The plan is to spend as little money as possible. If I can run my whole operating system on a 32gb ssd, then that would be awesome. I will probably buy a case to use my current HDD as an external hard drive. If I needed to store a bunch of large files I would have the room for it.
To answer your question definitively, For a linux distro (Ubuntu, in your case) With a few pictures and an external storage drive, 32 GB would be plenty.
You COULD run on a 16GB, but I would go 32, just in case.
I'm not sure how you define "fine", but my Lenovo with a 64GB SSD has practically nothing installed on it and I still have to keep an eye on it not to fill up. Sure, it runs and all, but it's definitely an unnecessary annoyance.
Back in the XP times, I had an 8GB HDD for years... it was horrible, but I even managed to have GTA SA installed, full install, nothing cut out of it, which is close to 6 gb... But right now, on Windows 7/8 I need 40gb minimum and 60gb to be comfortable with my OS partition. You seem to be talking about 64gb as your only HDD/SSD in the computer though, in which case I have to agree, that is way under the comfort zone for Windows.