Go to the layout and enter layout mode.
Select "page breaks" from the View menu
Go to print set up and specify the correct printer, page size and page orientation (landscape or portrait).
You will now see heavy black lines on your layout, one horizontal and one vertical (the horizontal line will only appear if you make the layout tall enough to exceed the height of the specified page.) Anything to the left of the vertical page break will print. Nothing to the right of it will print. Anything below the horizontal page break will print on a different page. You can size the layout parts to be just less than that horizontal page break.
You may also want to go to the printing tab in layout set up and specify margins for your printed page. These will move the page breaks in to keep anything from being printed in the margins and will produce a layout that will likely print the same on different printers. (How close to the edge of the paper you can print can vary with both the printer and the printer driver used when printing.)
Is there a way to lock this size? If I copy/paste a component, it will extend the page size- which I do not want (I want to force the page size for in case any odd reason. Will make things easier to debug if such a reason comes. Plus, it is annoying getting the error message saying that the layout size will increase with a duplicate/paste- and not give me an option to paste without extending)
Or is there a way to lock the layout to only allow a single page of contents (Disallow growth to another page..ect)
No such options exist and you aren't changing the page size, you are changing the layout size. Best I can think of is to paste as close to the top of the layout as possible, off to the right of the layout edge if you need open space and then drag the pasted object into position after pasting. (Click before pasting to get some degree of control over where a copied object will paste.)
Phil is correct, no "lock".
You might create custom Screen Stencils:
to help you with whatever size you need for printing (based on Zoom level in Page Setup, too).
An 8 1/2x11 paper with 1/2 inch margins all around will have these dimensions:
so I create a Custom stencil for it.