Justiciar
You'll be delighted to know that you're not alone. Getting the partition sizes right
a priori is something which few people get right every time once the system's been in use for a while, even in a commercial environment. Luckily, there are ways to get out of jail if things go TU, as you found out - du -k|sort -g, then rm that big download from r*dtube
I suggest that you keep gparted.and the system rescue cd in your toolbag. You're able to boot from these and resize if you need to. If you Giggle "linux disk space monitor script startup" you'll find a wealth of scripts to warn you if your space is running out.
Linux isn't for everyone - even not, as I suspect, Mrs SO. But as I have to fix the confounded thing when it breaks, she's crossing over to the dark side this weekend. Then I'll install Linux.
SO