Can I compress my entire user folder to make fit onto ext. HD?

from Disk utility help:

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To archive files in a disk image:
Move all the files to archive into the same folder.
In Disk Utility, choose File > New > Disk Image from Folder.
Choose the folder you want, and click Image.
Type a name for the disk image, and select where to save it.
Choose “compressed” or “read-only” from the Image Format pop-up menu. A compressed disk image takes up less space, and a read-only image is quicker to create.
To encrypt the disk image, choose an encryption option from the Encryption pop-up menu.
Click Save.
if you chose to encrypt the disk image, enter a password in the dialog that appears.
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Kj

This isn't going to mess up your user folder - you'll tell Disk Utility to create a compressed disk image on your empty drive and the computer will begin churning away copying files from your user directory and compressing them onto the external (I assume) drive. The only thing that could go wrong is that you might run out of space on the external drive and the process quits with an error - but that will have no affect on your user directory.

If most of the space in your folder consists of audio, pictures, or video, it won't compress very much since those file types (jpeg, mp3, etc.) are already compressed.

Those calculations are just guesses, and for them to be done it'd have to analyze each file. I guess it's possible to do a dry compression run to see what "would" happen, but that's not been implemented yet. Perhaps there is a command line utility that will do this?...dunno.

I have a lot of tiffs, will they compress more?

It depends on the TIFF. Some TIFFs are already compressed. VueScan, for example, has a compression option for TIFFs. Unlike JPEGs,TIFFs use lossless compression. A black and white (not grayscale) image may compress by more than a factor of 10. Compression should be done by something that knows about the graphics format, such as Graphics Converter.
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Using ZIP or Disk Utility's compression will not be as effective.

Kathykate wrote:
I have a lot of tiffs, will they compress more?

It depends, and is hard to predict:

If they are uncompressed TIFFs, they should compress quite a bit.

If they are already compressed, they probably won't compress any further

TIFF supports many compression formats, and some have different results with different types of TIFFs. When saving from Photoshop, for example, you can save with LZW, ZIP, or JPEG compression inside the TIFF. One weird gotcha is that if you have a 16-bit TIFF and apply LZW compression, it might get bigger! 16-bit TIFFs compress better with ZIP.

Frankly, the documents that will compress the most are text documents, and images that are not already compressed in some way.

Kathykate wrote:
It would be useful if there was a function to tell you what size a folder would compress down to, then you wouldn't have to waste time running it only to find you run out of room.

Determining that would take just as long as actually compressing them, so there's not really much point.