DVD Architect is not very good at estimating the eventual size of your DVD project. I can't recall if the technical reasons have ever been explained to me, but if they have, it either went over my head or I've forgotten. And honestly, by this point, I don't care. The estimates are rarely important for my workflow.
Anyhow. Before I go into more details about how to be reasonably (98%) sure that your project will fit on a disc, tell us more about your situation.
1) Which version of DVD Architect are you using? DVD Architect Studio 2.0 or 3.0? DVD Architect 1, 2, 3, or 4? (The former comes with Vegas Movie Studio, the altter with full-blown Vegas.)
2) Are you adding MPEG2 files to your project, or are the videos AVI or some other format? If they're MPEG2, got to File -> Optimize DVD and tell us what you see on the left. Where do you see checkmarks, where do you see yellow diamonds with exclamation points?
3) How much video are you wanting to put in this project? Twenty minutes? An hour? Two hours? This will determine what bitrate you want to encode at, if your files aren't already encoded to MPEG2. (There's also DVDA's Fit to Disc feature, again, if the files aren't already MPEG2s.)
4) Are you using single layer DVDs or dual layer? If they're single layer, you have about 4.3GB to use. Dual layer, somewhere around 8GB or so.
With more infofmation, you should be able to help you get around DVD Architect's estimation issues.
You can let DVD Architect do the encoding. Add any files you need, make whatever adjustments you want to the project, and then go to File -> Optimize DVD and hit the button marked Fit to Disc.
If you did encode it elsewhere first, I'd recommend using Vegas rather than TMPGEnc. And you would want to use a bitrate calculator to determine the best bitrate for a video of that length. DVDA's Fit to Disc feature takes care of that by doing the calculations itself.