Machine heads come in a varity of different styles and sizes. The three major catagories are classical for nylon strung guitars, bolt on for electric and accoustic steel strung guitars, and bushing based which can also be found on accoustic and electric guitars. Companys usally stay to a standard measurement for each type of machine head when designing the dimensions, this make replacement parts easier to source and fit.

Classical

Found on nylon stringed classical guitars measurements for these are usually the same. So far every classical i have worked on has had the same dimensions. I will upload a diagram as soon as possible. 

Bolt on

Bolt on machine heads are found on acoustic and electric guitars, they typically have a diameter of 10mm (all my bolt on machine heads are 10mm). Bolt on machine heads usally have one fixing scew to prevent the machine head from turning, unlike busing based heads which have two screws which are there primarly to hold the head on. Bolt on heads as the name implies are held on by a bolt which enters the front of the headstock and scews into the machine head on the other side. The bolt is usally a hex nut, so if your heads have a small hexagon/octagon on the front try unscrewing this with a spanner to see if it removes the head, if it does you have a bolt on machine head.

Bushing based

Bushing based machine heads are so called because they have a bushing ie a metal insert running through the headstock. The tuning peg slots into this insert and rotates within this bushing as the guitar is being tuned up. The bushing part should typicaly fit into a 8-8.3mm hole and the peg is usally 6mm. manufactures all try to keep these measurement the same so parts are interchangable and a standards is set. These heads are attached via two screws to the back of the headstock.

Hopefully I will have some photos up soon to clarify the different types of machine head.