There should be a few inches of allowance at the hips, since this is the primary area of motion – and thus tension – in the garment. If the hips are too small the pockets will flare and the fabric will show stress lines (or “pulling) across the front of the crotch and the back of the seat. It will also be difficult to fit your hands (or objects larger than your cellphone) in your pockets. If the hips on the trousers are too large there will be extra fabric “pooling” on the side seams and at the center back seam.
The “rise” of the trouser, i.e. the distance from the top of the waistband to the “4-corner intersection” below the crotch, should follow the natural shape and size of the body. If it’s too small, it will feel like you have a “wedgie” (which can happen in the front or back), will feel uncomfortable to keep the pants on your natural waist, and the front of the trouser may be too “revealing” (especially if the hips are cut too trim as well). On the other hand, if the rise is too long, there will be an extra area of fabric hanging below the crotch and impeding your long strides, like a “drop crotch” pant would have.
This is where a lot of guys make the mistake of going too trim. We appreciate that you would like it fitting snug when standing up, but you should also be able to sit down comfortably, without the trouser fabric struggling to keep it together. If you notice a lot of fabric stress (wrinkling/pulling) at the upper/inner thigh when you sit, your trousers may be too snug. This will greatly affect the lifespan of your trousers, especially if you’re sitting at a desk all day.
The hem width of a trouser should follow the look and feel of the rest of the trouser i.e., if you are looking for a straight cut trouser the hem width has to be cut wider than if you were tailoring a tapered trouser. For those with quite large calves, we are slightly limited as to how narrow we can go, otherwise you would be constantly pulling your trouser down whenever you stand up.
We've all seen the trousers that are just too short and showing a too much ankle. Whilst I appreciate that there is a time and a place for short trousers e.g., on a beach, that's pretty much it. The bottom of a trouser should always be cut at a slight angle so that it sits just on the top of the front of the shoe, but goes over the back of the shoe to ensure that not too much sock is shown when walking.