I went looking for some useful pix and found it very difficult, compared to finding pix of shirts NOT fitting (shockingly common, even amongst fashion pix supposedly showing off a nice looking shirt...AND very instructive)...or of people looking good in shirts whether they fit or not, which obviously is a matter of taste or dispute. There's plenty of those!
For developing your own standards with help from a very discerning eye, you could hardly do better than to study this post. And, I'd say, to also keep doing your own on-going and never really ending survey of what looks good—AND bad—to your developing eye. Collecting pix will increase the usefulness of your search, as you'll actually be able to see your own standards developing over time.
I snapped a bunch of screen shots at Pinterest, but quickly found I had so many (almost all negative) that it would be quite time-consuming to put them all together with even the most minimal commentary, but I did find it fascinating to see how obviously uncritical of fit either most image posters are, or most fashion stylists are, or both. Suggesting that hardly anybody seems to really know what fit looks like in a shirt, or that the standard for shirt fit is quite low, not really surprising since almost nobody wears custom-fitted shirts, but almost everybody wears some sort of shirt, and hardly anybody, not even models, has a perfectly "normal" shape. Also of course, whatever style is current is a highly moving target, except that no shirt EVER seems to be tight enough for slim young guys, in any era!
And finally, in that same vein, the main thing the average image-posting person seems to be imaging that male shirt fit is all about is not actually a fitting garment, but a "fit" body inside it, being shown off, often by how much it's straining and distorting the helpless garment because it's such an extreme and shapely form.
The big question I came away with was: What's the difference between bad fit and simply too tight in an era when too tight IS the current fashion? My own opinionated answers were simply: 1) No matter how tight, the yoke, shoulders and upper body, at least in front, should be as perfectly smooth as possible, with no drag or sag lines going in any direction around the neck, or between the shoulder ends and the neckline and center front. VERY rare! 2), and even more rare, there should be no diagonal drag lines across the side seams; almost every slim male body seems to suffer from this, esp. from lines going from up in back to down towards the front. And finally, and rarest of all, 3) the cuffs, when buttoned, should be right at the wrists, not falling over the heels of the hands or exposing wrist bones, while allowing for some drape in the sleeve.
I'll post my images sooner or later, because I think your question's an excellent one, and that here's a perfect place for a gallery of opinions on this topic, and presumably a few fitting "facts", too. but right now's a VERY busy time, sorry!