JacketS
The circumference: You can add up to an inch and a half in circumference at the waist of a jacket (Assuming enough seam allowance; talk to your tailor). You can add less at the chest (because of the armhole) and hips (because of the vent). Be aware, this mostly affects the fit at the back and sides of your jacket. The front side seam probably has a pocket going through it and thus can't be altered. Likewise, taking in a jacket is also constrained by the armhole and vents but you can take it in up to three inches or so. HRD wouldn't recommend taking much out at the hips of a jacket, as slim as your hips may be, for ease of movement and to keep the visual balance with your shoulders. You should be able to take the hips in or out an inch if it has English vents (2 vents), and up to two inches if it has an American vent.
Though it probably doesn't make much difference, it can make a world of difference if your clothes are still looking funny: the vertical placement of the waist and hips. The cute and compact are most likely to find their jacket waists are far lower than their own waists and thus all their jackets continue to look over-sized despite tailoring. If your jacket looks too big at the underarms or if the collar is too long, it's probably the vertical placement of the waist. Note: it can also be the width across the front of your chest (the armhole should sit just past you shrug your shoulders). Talk to your tailor about taking more out at the seams above your waist and not just at your waist.
Shirts
Tailoring a shirt through the torso is a straight forward affair. If there are darts on the back, you can take an inch or so in there. You can take the shirt in an inch or so there even if there aren't existing darts; darts are easy to make. If you have a vent in the center back of your shirt, this may affect how much your shirt can be tailored in the back. Pleated vents let fabric move and darts pull fabric in and while this sounds like vents and darts should get along like twisting and shouting, it's more like the dart can pull the fabric too far out of alignment and the vent will let it.
Fortunately, shirts can also be taken in along the side seam. If you're just taking a little out of your shirt, do it at the darts in back. The underarm and side seam of your shirts have a nice flat felled seam and many tailors won't replicate this. So, keep it if you can. But if you want to take more out of the torso, or even more likely, if you want to take out of the torso and the underarm, tailors fake a flat felled seam very nicely. It looks almost the same from the outside and is almost as long-wearing.
Of course, jackets and shirts can be made shorter from the hem. They really can't be made longer. Jackets can be made shorter by just about any length and can make a big difference in the overall look of a jacket. There is usually an ideal place for a jacket hem to hit, traditionally at the knuckles and contemporarily between the top of the hip bone and hip socket. If your jacket fits well but just doesn't look quite right, the hem may very well be it. Shirts can also be shortened. More interesting, the hem shape can be changed. Talk to your tailor if you want a different hem shape.
