If by "2.5"" you mean closed OAL, then just get a Case or Victorinox that suits you. If you want something more "high-end", then look at a William Henry Knives "Kestrel" w/a damascus blade.
If however you want something with a ~ 2.5" blade, then definitely consider a Spyderco "Lil' Native". Extremely easy to deploy, and has a very solid locking mechanism. Bonus points for being made in the US. Of course a Spyderco Delica or Dragonfly is always a decent choice too, but I don't care much for lock-backs.
For lower-priced knives, if you want a thumbstud knife, take a look at the many iterations of the Civivi "Baby Banter". This is one of my favorite low-end small pocketknife. The jade G-10 scales are easy to dye if you want to customize the color. Personally, I like the black blade/purple G-10 and the CF/damascus blade versions. The Civivi "Pragma" might also be worth your time to investigate. If you want a low-end small flipper, then consider a Kizer "Lieb". These have excellent flipping action for such a small knife. Also the "M" version of the Kansept "Korvid" is a bit more unusual design that might interest you. I haven't used the M version, but I do have the small version and it is a solid knife that I carry in my jeans watch pocket.
At the mid-high end, there really isn't anything in the ~2.5" area that I would recommend. At the high-end, I can recommend a Medford Knives "Dress Marauder", which is an insanely expensive and insanely well built small knife. These might not be in production any more, but it really is a jewel of a knife. Be warned that the owner and his opinions are evidently quite polarizing to many (I couldn't care less). There is also the Chris Reeves Mnandi which is also built extremely well, but does go over your blade length limit by about 0.25". A William Henry "Lancet" would be really over the top, especially if you get one with a damacus blade and mammoth tooth scales. Lastly, you could contact Curtiss Custom Knives and have him build you a custom "F3 Compact", which is also an insanely well-built pocketknife. You can see these at the DLT Trading website. Be aware that these are heavy for their blade length, but the workmanship is worth it.
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