Dual monitors at both work and home....

Home:

2560 x 1440 - 2 x 32" HP

Work:

2560 x 1440 - 32" HP

1920 x 1200 - 24" Dell

I always try to use native resolution wallpaper per monitor

I usually get the largest size available within reason.  Provides a bigger canvas to work with for cropping, resizing, etc.  Yep, I play with my food.   

3440x1440

Is this a combined res, alphabyte?  Or a single monitor?

I also use 4K, on my 4K TV.  There are lots of 4K walls here.

3440x1440 here, too. 1 Monitor LG Ultrawide.

Single monitor, ultrawide screen. Was skeptical at first whether or not this format would work for me, but really love the extra space for multiple windows.

I have one of those, I don't even know the res....lol.

EDIT: 2160 x 1440.

1920X1080 on a 24" LG

Just a quick tally so far.

1920x1080 - 5

1920x1200 - 4

2560x1440 - 7

2560x1600

3440 x 1440 - 3

3840 x 2160 - 2

Another 1920x1080

I guess to answer your question sizes, I do look at resolutions available. when ya jump up to 2560, a 1920 wallpaper will be fuzzy. So yes i look.

But still the same ratio.   I use 2560x1440 and I really don't see a major diff when running a 1920 wall.

Follow-up question:

When you download a wallpaper do you prefer multiple resolutions, or just the biggest possible and fit the wallpaper from that?

I definitely prefer multiple resolutions. One of the reasons is that sometimes I like the possibility to crop the larger size for using only portions of the original.