Normally you would click on the page in the main window and when you see the yellow outline click on the zoom slider at the top of each cell. Try dragging left to zero%
This is actually rather simple, if, of course, I have understood your request accurately.
It is possible to both control the zoom and the frame size on a page.
Lets say you want the entire image on the page at a size that is different from the default.
All that is required is to select a particular page by clicking on it.
You will know it is selected by the yellow frame around the page.
Next, hover the mouse over the edge of the image as it exists on the page.
Try both the horizontal and vertical edges as required until you get a change in the mouse cursor that resembles a cross with arrow heads.
L-click and then drag the image around until it is the size that you want on the page.
The image stays centered on the page.
The aspect ratio of the image is not altered.
If you choose, there is an additional control to zoom the image and then move the image around, within the frame, to get the framing that you desire.
See how you go.
You nailed it, deepakgahlot! Thanks ever so much! Just tried it and it worked like a charm.
I just wish I could have figured that out by myself but Lightroom Help is singularly unhelpful. I guess LR thought, "Not Adobe's problem, it's Blurb's" and Blurb said in their email to me that it's not their problem it's Adobe's!