garage siding and window replacement

I have a detached garage that I plan on having the siding replace in the near future. It also has a window that needs replaced. This is the only window on that side of the garage. Because the size of the window (23" x 23") is non standard, I would rather install something that Lowes has in stock.
Can I simply reframe the opening to a larger size and install the new window? Any info on the internet about reframing window openings or is there not much to be concerned about?
Currently the siding has 'paper' underneath it. I understand that it would be wise to install OSB. I would like to remove the siding ASAP so I can install the window, but it will be a few weeks before siding will be installed. Would it be ok to install OSB and leave it exposed to the weather for approx 4 weeks or should I cover it with plastic or something?
Many thanks, -j
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I would visit www.Pella.com and/or andersen window's site to see the importants of properly flashing a window. Buying a new window and not installing it correctly is a waste of money. You can download installation instructions at those sites and see how to install a quality window even if you buy a different brand.
You say there is paper under the old siding. What is under the paper?
The right grade of OSB can take a fair amount of weather.

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Nothing is under the paper, just studs. (Actually, a portion of the garage contains a small office and it has insulation under the paper).

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You need to install the right grade of OSB and then building paper, like tyvek, over that. Does it have footings?

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Excuse my ignorance, but I don't know whether it has footings or not.

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