Find

To find a file changed in the last X days
The following prints a file changed in the last day

find. -ctime -1 -print

Also see ctime.

To find a file accessed 30 days or before
To see files that were accessed last 30 or more days from now you'd do

find. -atime +30 -print

Also see atime

Large files
find. -name '*' -size +500M

will search from the directory you're in, recursively, listing all files over 500M in size. If you're not root, you will likely get a lot of "Permission Denied" errors if you are traversing a large file system. You can avoid this by redirecting all errors to /dev/null:

find / -size +1G 2>/dev/null

will search every file under the root directory (/) for files over 1 gigabyte, ignoring all error warnings