Most machines use a standardized portafilter basket with the volume capacity of 14 grams for the 'double-shot', but 12 gram versions are available for some machines. For the 'single-shot', most people are comfortable with the standard 7 gram basket, not even knowing that a 6 gram basket is available. Some machines portafilter bodies are able to accept an even larger volume (read: taller) basket: a 'triple-shot' or 18-21 gram basket.
The over-all dimensions of a particular basket vary depending on a number of contributing factors. Some manufactures chose to use their own proprietary design for their portafilters, and so only the baskets manufactured for that particular brand will fit in their portafilter. The measurements of a basket (or portafilter) is in a way in non-issue. It should be noted, and cannot be stressed enough, that it is the size of a tamper base, be it 45 or 58mm, that is measured, and not the portafilter or the basket itself. The most prolific manufactures on the market use portafilter baskets that accept a 58mm tamper base. The rest (with one, hardly needing mention, exception), are smaller.
Baskets are manufactured using a large sheet of steel and a press with a stamp mold. This means that over time the stamp mold can wear, creating variances in the appearance of the basket. Depending on which factory a basket was stamped at, due to the design of the mold for the stamp at that particular factory, baskets can have a number of different parameters. Most importantly are the outside diameter and height. Too narrow and the basket falls out when knocking the ground coffee puck out of the portafilter. Too tall and the basket won't fit in.
But there are other factors as well. In order for the baskets to stay in the portafilter, they are designed to work in conjunction with the portafilter spring. This spring is inserted into a slot or run on the inside of the portafilters. The slots are machined at a certain height from the top of the portafilter body. On the baskets are what are commonly referred to as "ridges". These ridges are formed from the stamping process and are designed, based on the mold for the stamp, to interface with the spring and hold the basket in place. Depending on the stamp that is used to press the basket, the ridge on a basket will vary in height from manufacture to manufacture. They may also be more or less pronounced, and can vary in width. In addition to this, the rim of the basket may vary as well. The surface of the rim can have different degrees of curvature, meaning that when the portafilter is engaged in the group head, where the portafilter is finally positioned at may change in orientation depending on the profile height of a baskets rim. Adding to the whole topic - way over the top into the realm of border-line obsessive - even the holes in the bottom of a basket are not the same in size or quality. At the bottom of the stamp for the basket are protrusions for punching holes through the metal. These punches eventually wear and get dull, leaving rough edges and or not punching all of the way through. The design of the punches can change too, in their hole pattern and size.
Describing all of these factors (and this ramblings in general) is meant to mean "no two baskets are ever alike", or as a disclaimer that although we strive to provide the best quality products, this is not always in our power.
In the end, what basket one feels like produces the best product is going to be the choice for them and, unfortunately, this is not predictable.