Can you use the large for a D850 with small lenses (in addition to long lenses)?

You should be able to. Just don't extend the snout piece as long.

What size cover will fit a nikon d90 slr

The large size is a tight fit on a D500. I never tried it on the D850 but I think the D90 is smaller than the D500. I use it on the Z7 with arca plate/grip extension. Don't have a D90 so don't quote me.

Will this fit my canon m50 with a rode video mic attached?

I do not use this item because it is awkward to remove when shooting. I do use other Peak Design quality products. I have shot only Nikon (D850, D500, Z7) for over 50 yrs. I don't even know what a m50 or rode mic look like. Okay Amazon?

Product Dimensions	25.4 x 17.78 x 6.86 cm; 0.15 g
Batteries	1 12V batteries required.
Item model number	housse Shell large
Special features	Waterproof, Ultralight, Anchor Ports
Other display features	Wireless
Form factor	Case
Colour	black
Manufacturer	Peak Design
Country of Origin	USA
Item Weight	0.15 g

Shell Rain Cover (Large)

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