I used to say this all the time when I sold binoculars for a living:
The jump from a $25 pair to a $250 pair is a much bigger jump than from $250 to $2500.
I feel like $500-800 is about the sweet spot for most people, but if you can put the money into a leica/swaro/zeiss you won't regret it.
For sizing, I like to have two pairs:
An 8X32 set that come with me everywhere. They're smaller, stable, easy to carry, and easy to "spot and point" with the 8 power. You see something, bring the glasses up, and you're right on it.
I also keep a 10X42 set for "sit and watch" moments. More magnification means it's better to be resting your elbows on something, larger glass for a little more light capability.
I've known people who use 12X and 15X units, but I'd only ever use that when tripod mounted really.
Similarly, I don't think I'd go smaller that 30mm for your objective lens. The smaller micro-glasses start to feel really pinched, even with the top-notch glass.
Brand wise, I don't think you're ever buying bad binos when you spend over $500.
The way I break it down is:
Under $500? Probably pick bushnell.
$500 to $1000? Probably Vortex or Leupold or one of the budget-Europe brands
$1500+? Time to go hard on the austrian/german and do some real side-by-side comparisons if you can.
When I had hours everyday to sit and compare optics at different ranges and light conditions, I found I preferred Swarovski Binos, Leica Rangefinders, and Zeiss spotting scopes.