The comment that EXIT PUPIL will determine the highest magnification you can take advantage of is correct.
The OBJECTIVE LENSE diameter divided by the magnification will tell you exit pupil size.
Lighting conditions will determine your eye pupil size. It will vary between approx 2mm and 7mm.
A common general purpose binocular might be a 7x by 35 mm. It would have a 5mm exit pupil. My 6x by 42 mm rifle scope is good in low light. It has a 7mm exit pupil.
I have 2 spotting scopes. One is a fairly high end Kowa with a 77mm Fluorite crystal objective lens . The ocular is a 15 to 60 x zoom.
I can typically see 22 cal holes in the bull out to 300 yds. If I use an orange on white sight in target, its no problem to set targets at 300 yards and sight in perfectly without going down range again.
When th light is just right (rarely) I can see 30 cal bullet holes in the black bull at 500 yds.
But I don't gain any better image above 40 to 45x. The 60 X does not gain me anything.
I have another spotting scope. Its an old Bausch and Lomb NRA model. 60 mm objective, Fixed power 20X . It was made in the 1920's.
With an orange on white target, I can count on seeing 22 cal holes easily at 200 yds plus.
IMO,using a tripod on a target that holds still, FOV is not a big factor, My KOWA has a raised "pointing bead" that helps find the target.
Just getting "more" of something (x's or mm) only helps if its reasonably good quality.
I would not go less than 60 mm objective, Go more if you can afford it.
At 300 yds,? speculating...I don't know of you wear glasses. You can get longer eye relief with the right 20X to 30X fixed eyepiece, or you can get a wide angle with more FOV.
There are tradeoffs to everything. A 15 to 45 X zoom is a good compromise.
I'd call 60 mm minimum. 66mm or 77mm ...even 80 mm are great but cost a lot.
I'm sold on Kowa.
I have no experience with ordering this way,but you can find good prices on E-bay ordering direct from Japan. I suppose you could get screwed,too.
If there is a club or group you will shoot with,go see what works for them.
If you will be shooting spotting for yourself, a 45 deg eyepiece is good
I like mine.