Hi
"In the preview settings: text clip width 800, height 450, what unit is this? is it in, mm, pixels, or what? "
This is the size of the preview image in pixels. If you click Tools>Options>Display. and then look at the choices in the Preset drop down menu you will see all the VP preset image sizes (resolutions) with their AR. Choosing one of these sets the Width and Height of the Preview Display. This can only be seen if the preview pane is undocked by clicking the arrowed box in the top right hand corner. Note that the larger this display is set the longer it takes VP to make any editing changes before it can be previewed and can result in the "Building Preview" message. However the image in the undocked window will have higher resolution.
If you never intend to undock the preview pane you may as well leave the display with the lowest values that have the same AR as your clip. (Not the resolution.) i.e. if your clip has an AR of 16:9 then you could set the preview to 512 x 288 pixels or 640 x 360. The preview and timeline images will look OK and you should find that processing goes faster.
The resolution of the preview display will not effect the export resolution as you set that in the export parameters.
What I mean by that is for example:
If your raw clip is an MPEG4 of 1280 x 720 pixels.(16:9) . You can preview this if required at 512 x 288 (16:9). This looks OK on the timeline and in the docked preview pane...as despite the lower resolution the image is smaller and appears clear. However if the pane is undocked and enlarged to full size it will appear blurry. As mentioned above if you are intending to undock it to do more precise editing then a higher resolution will make the preview image clearer (but processing any editing changes may take longer with possible "Building Preview" warnings. A lot depends on the PC.)
Even though your preview is low (512 x 288 16:9) It's only for previewing. it won't affect your export and you would export the clip for example as a 720 MPEG4 using the highest quality setting in the export windows. The result will have the same quality as the original clip.
Note you say your clip is 800 x 450 i.e. 16:9 (AR =1.777) so either of the settings mentioned above should show the clip on the timeline and in the preview screen full frame with no borders reasonably OK. You could also directly enter these values as a custom preview setting. For exporting you can set the format in the same way under the Custom option or use the Preset Detect option.
Nat