QT File size for DVD Studio Pro

You are approaching this incorrectly, I believe.

File size isn't the issue.

The number of total minutes of video is the pivotal factor.

Keep the source videos outputted at Export/QT/Current Settings/Audio and Video.

If you accumulate less than an hour of total video in minutes, set your video encode rate to under 7.4 mbps and AC3/Dolby 2 for audio in Compressor.

You can get about 75-80 minutes of 7.5mbps MPEG2 video onto a DVD5 (4.37gb DVDR).
But to get to a DVD, it has to be in MPEG2 and MPEG2 is compressed to like 5 to 1 compared to your original DV file.
45 minute DV file is 10gb or so.
45 minute MPEG2 file at 7.5mbps is around 2gb or so.

You will be fine using Compressor with a 60 minute video.
When you get past 80 minutes, you have to start dropping the 7.5mbps bitrate accordingly.

90minutes of video at about 6mbps sounds about right. The file should be able to fit onto a DVD5 (DVD-R) of 4.37gb.
But you are saying the file size is still 5gb?
It didn't change from the 7.5mbps version?
That doesn't make sense so something must be set wrong.
Have you imported this 6.0mbps version into DVD SP.
Click the file in the finder and select APPLE + I.
This will show you the file size.

Make a new Preset in Compressor and make a new MPEG2, 2 Pass VBR Best, 5.5 average, and a 6.0mbps maximum.
Just to be sure you can get it to fit. If the file turns out to be like 4.1 or 4.2gb, then up your bitrate a bit if you want a bit more quality.
Just do some experiments.
When you drop the bitrate or up it, the file size WILL change.

If you have 4mbps average and 6mbps max, you should be able to get 90 minutes easiliy. Are you compressing your audio as well? You should use Dolby Digital, Stereo, 192kbps - 224kbps, and -31 normalization. If you have a 60 minute movie, the uncompressed AIFF file is 700mb roughly and Dolby Digital would be a 70mb file. About 10x the compression.
Plus the audio still sounds good.

You can just use the Compressor presets for 90 minutes of footage and it should be fine so if you are over with 4mbps average and 6mbps max, something is wrong.

Anything below 4 is pretty bad. I never really go below 5 even.

Just delete the .aiff file out of your DVD SP project, but don't delete the actual .aiff file. Drop the .aiff file into Compressor and then use the Dolby Digital settings.
This creates a .ac3 file that you will import into DVD SP.
Then you should be able to use a higher bitrate to get 90 minutes of footage onto your DVD because 4 is way too low.

Use this calculator...

http://www.videohelp.com/calc.htm

This helps with determining bitrates.

You can compress with any settings you want.
You either use AIFF OR Dolby Digital in a DVD SP Project, never both.
I would highly recommend to NOT use AIFF in a DVD SP Project.
The bitrate for AIFF is 1.5mbps and it is just too high.
Use Dolby Digital in a DVD SP project at like 192kbps or 224kbps.

With that said, this is a correct workflow.
Export your self contained .mov file out of your editing program like Final Cut Pro (45min = 10gb estimate)
Quit FCP, open Compressor.
Drag your .mov file into Compressor.

Now there are many ways to encode, but to do an easy test, just select the presets for 90 min encode for MEPG2 video, and Dolby Digtal for audio.
After it finished, you will have the .m2v file for video and a .ac3 file for audio.
Then import those 2 files into DVD SP and author your DVD.

The difference between Max and Fixed is the degree of flexibility you are giving the Compressor to vary the Bit-rate, the Max is how High it can go - for fast action, and this difference will also be used in the other direction, to Low levels - when there is little movement.
Like swinging a ball on a string. The shorter the string, the smaller the arc. Does this help?