Depends -- which version are you using, and are you authoring a DVD or a Blu-Ray disc?
DVD Architect Pro 5.0 is capable of authoring Blu-Ray discs, which allow for formats larger than 720x480. But of course you need blank Blu-Ray discs and a Blu-Ray burner.
If you're just authoring a standard DVD, as johnmeyer says, 720x480 (for NTSC; 720x576 for PAL) is the limit of the DVD format, not just the limit of DVD Architect.