How to email an iPhoto slideshow, size of which is 50+ MB.

I can not imagine how QT Pro will help you. I don't believe that there is any way possible to reliably send such a file via email. To my knowledge, the only solution is to rent server space, such as dot-Mac but, of course, that's US$100/year.

Most ISP's place file size limits on email attachments and they are different for your recipients, too.
QuickTime Pro ($30) is a tool that can "export" (re-compress) your file and dramatically reduce the file size by adjusting the things that make a file so large to begin with.
Just changing from 640X480 (probably your current size) to 320X240 (the size used in most Web pages) would reduce it by nearly 4 times.
Changing the video codec and data rate could make it smaller still.
Iphoto 6 takes your image files and turns them into "video". This and your music make the file so large.
QuickTime Pro is also an authoring tool and it could make your slide show. I use QuickTime Pro to make all of my Web based files. Only a few are over 4 MB's in file size.
Some of my links that showcase these QuickTime Pro authored movies:
http://homepage.mac.com/kkirkster/03war/
http://homepage.mac.com/kkirkster/RedneckTexasXmas/index.html
http://homepage.mac.com/kkirkster/B/index.html
Having a Web server is another great way to share large files. You need only send the "link" in the email.

Import your slideshow into iMovie. Make sure it is in the timeline and then choose "email" from the "share" menu. iMovie's email default settings will give you a decent size file without too much folling around inside of quicktime.

That is a workaround and it doesn't take much time.
But the file exported from iMovie to "Email" will only be a 240X180 QuickTime movie.
Kinda tiny for images.

You're correct. It's not 4 MB's as I stated.
It's 3.73 MB's.
http://homepage.mac.com/kkirkster/.Public/RedneckTexasChristmas.mov
Copy (don't click) the URL above.
Open QuickTime Player and then "Open URL" from the File menu.
Paste and click OK to download and view.
All of the transitions are based on QuickTime and are not rendered as "video" which keeps the file size down. The audio is about one third of the total file size.

Kirk:

I've been able to create a sequential slide show in QTP and add music but am stymied when it comes to adding transitions. How does one do that as you described?

Do you Twango?

Pretty cool trick, isn't it?
Still images acting like "video" without the file size bloat of video codecs!
Most of these effects and transitions have been around since QT 3 and a few examples are still found in the export dialogs of QuickTime to QuickTime Movie.
It seems Apple and QuickTime designers and engineers live on different sides of the planet (QuickTime, Claris (now FileMaker) and some others where located in upstate New York). I don't know where they live now and maybe they are not talking to each other.
QuickTime develops QTVR (version 3) and Apple creates software to enable users to make their own. And then it seems to stop and no updates to those tools.
QuickTime version 5 introduces "skin track" movies. But Apple (minus some out of date tutorials) seems to have left development to third party vendors.
I make QuickTime files that challenge the viewer "How did he do that?".
So. To answer your question, here are the steps I used:
Size, edit and adjust your images prior to opening them in QuickTime.
Open the image (new Player window), select all (Command-A) and Copy.
Open any audio file and extract just ten seconds of audio (the time is your own choosing) into a new QT file.
Select all and choose "Add to Selection & Scale" (version 7).
Now your single image has "time" (but it is still file sized as a single image).
Movie Properties window allows you to highlight (single click on the video track) and then "Extract".
New ten second "video" of your still image file is made. Save As, name and "self-contianed".
Repeat for all of the other images you wish to turn into "video". Time can vary but I've found some of the special transitions require at least 10 seconds.
So now you have a folder of a bunch of ten second "still image" videos. Each QT .mov file is measured in KB's (because you sized and edited prior to import).
Last "trick". I promise.
It involves very old QuickTime software that was never released for general use. And it (then) only worked in OS 9. A "newer" (PPC version) can be found here:
http://developer.apple.com/samplecode/samplemakeeffectmovie/index.html
and it works on OS X.
Open movie A (use the "Test" menu) and navigate to your folder of new "videos". Pick one and click OK.
Same dialog will now open again. Pick movie B and here is where the transitions, compositors and wipes can be used.
Use Save As on these new "movies" and join them together using copy/paste.
Add them "scaled" to your audio track (extending or reducing) the effect used.
Lots of fun. Very tiny files that you can re-purpose with special html page code to do lots of "tricks".
QuickTime 7 and Tiger have introduced Quartz Composer effects that make my old stuff shame in comparison.
http://homepage.mac.com/kkirkster/.Public/Tiny_Movie.qtl
29 KB's.
Glad you asked. I have only sold 4 copies of my PDF in the last 6 weeks.