Replace shower window when redoing interior vs exterior wall?

[Sukhoi]

This winter I plan to redo our hall bathroom, which has a window in the tile wall above the bathtub. Eventually we plan to redo the exterior siding as well, but not this year. The currently installed window is a 20+ year old replacement, and I want to replace it at the latest when the plywood siding is done (would be doing other windows at that time).

1. Should I plan on doing it now when I do the shower tile? I realized I don't want to tile to the current window then have gaps or not enough space if I later replace the window with the siding.
2. If I should do it now, any thoughts on if I should buy a replacement vs new construction window? Is the weirdly large window frame & trim likely to be covering enough gap in the plywood siding I could get in the flange of a new construction window? The window interior has some vinyl strips I could remove for additional inspection if needed. It's also worth noting the window is very protected from rain by the roof eve, so the waterproofing doesn't really need to be perfect. It'd be fine to slide a new construction flange up under the plywood and not properly do it until later when the siding is replaced.
3. For either type of window what exactly am I measuring to choose the new window size to order?

 

[Paperdoc]

I am facing a similar issue for near future. I really think we do NOT need a window there. So it WILL get removed. Your issue is: when? I would do it now when you redo the entire bathroom. It would not be heard to install some exterior covering now that can be covered again later whenever you do the exterior siding. If you delay, though, you WILL have to figure out how to fill in that window hole inside to match the recently-redone rest of the bathroom. That would be difficult.

 

[Greenman]

Do the window at the same time as the shower, use a new construction window. That simplifies the flashing details and you won't end up trying to alter a tiled wall later.
When you address the siding at a later date use pneumatic fasteners or screws so you're not pounding on the back side of a tiled wall.

 

[Sukhoi]

Alrighty, will definitely do the window prior to or when doing the shower. Now I'm thinking about just doing the whole exterior project first as the front door and single-pane front windows are really starting to show their age. We've always done work out of the ideal order on this house and maybe it's finally time to fix that.

The room is way too dark to remove the window, which is annoying as I hate windows in showers. Also good call about not hammering on the exterior side of the wall...I had never thought about that.