Even if you hate everything she picks out, buy at least one of them, be it a shirt, pair of pants, shoes, etc. She’ll love you in it every time you wear it, and that’s priceless.
With regards to the blazer thing- Try navy blue, but have a tailor or dry cleaner swap out the brass buttons with something that doesn’t look so Caddyshack while you have the sleeves tailored for length. Navy blazers work in more situations than almost anything else and they go with more of your clothing than black or grey will. Also, don’t always go with the American Sack look where there’s no padding in the shoulders at all. I’m not saying go with supersize 80’s shoulder pads, but a little help makes the jacket look sharp and less boxy. Sportcoats (blazers) should look sporty, not frumpy.
Grab an issue of GQ or Esquire and look for styles and looks that you like. Then go out and replicate the look. Notice things like patterns, weaves, cuts, and fits. There’s really almost nothing new under the sun, so anything you see could usually be found similar in used clothing and updated as needed with some inexpensive tailoring.
I’m shaped similar to you, if a little shorter, and I gotta say that if you feel awkward in really tight clothing then don’t be afraid to layer shirts. I have a real problem finding short sleeve shirts that fit right on my frame, as they either fit across my shoulders or down my chest, but never both. I also find that most t-shirts don’t go far enough down my arm and end up looking more like sleeveless shirts. The solution? Layering short and long sleeve shirts. It lets me look much more casual than a button-down, while still not being forced into the quandry of looking like a really frail hulk or a puddle of cloth. When doing this, I normally buy the long-sleeve one size bigger than the short sleeve.
Just an idea.
Oh and I must re-emphasize NEVER STOP YOURSELF FROM TRYING THINGS ON. If you see a new style or fit that’s roughly in your size, go to a dressing room and check it out. Don’t like it? All you’re out on is time. It prevents your look from growing too stale. I would recommend taking someone you trust the first few times, but after a while you’ll begin to know when you look good. It’s a skill, just like any other, and such self-awareness needs to be trained.
If only because you’ll give your girlfriend the opportunity to point and laugh - seriously, if I’m shopping with friends I’ll regularly try on stuff in my size that I know is going to look terrible, just to crack them up. Secondly, it does give a great idea of items that WON’T work for you - if you see a similar item in the future you know you can give it a pass - but make sure to try it at least once.
Regarding colors, with your coloring I think I would take a pass on black - it will wash you out and look kind of cliche, if you don’t mind me saying. A tall, thin guy in black is totally obvious - dark brown would be much more flattering.
Regarding fit - I agree with everyone else - you’ll up your style cache by 100% just by getting better fitting clothes - oversized tops/jackets give you that “I’m a college student with no money and no idea how to dress myself so I chucked on whatever was handy” look, which from your OP is not what you’re going for.
There’s a guy on my floor who’s built quite a bit like you and he wears tighter fitting tops and looks totally hot. Thin guys tend to have really attractive definition in their shoulders and arms - show that off!
Someone mentioned changing the brass buttons on a navy blazer. I’d say skip that step by buying blazers at places like Urban Outfitters or H&M - they sell blazers that are easier to wear in a more casual way and they probably won’t have ugly buttons. No need to go buy an old man’s sailor-looking jacket when they sell ones geared toward the younger crowd.
Hopefully someone can chime in with where your pants should hit when you are standing. And don’t bend over to check Use a mirror. But don’t get weirded out by some of your sock showing when you sit down. My brother has a thing about that so he buys nice suits but with the pants too long, so he ends up walking on the backs of them and shredding them.
I don’t know pants, but I’m pretty sure a sport coat is supposed to hit just longer (an inch maybe?) than the base of your thumb by your wrist. When your arm is outstretched.
OK, this guy says “The [shirt] sleeve length should be such that the cuff hits that bone at the top of your hand some 4 inches above your thumb. … Jacket sleeve length: men tend to wear them too long! One-quarter to one-half inch of your shirt cuff should show below the jacket. You should measure this standing in place.” " The length of [trouser, not jeans] leg should hit the top of your shoe. It should not break too much."
Use a mirror. But don’t get weirded out by some of your sock showing when you sit down. My brother has a thing about that so he buys nice suits but with the pants too long, so he ends up walking on the backs of them and shredding them.