Getting the right size is really important, especially for knicks, bibs, etc, and choosing the size can be a real pain-in-the-you-know-what because sizing varies between manufacturers, and sometimes within a brand! (I'm not joking when I say if I could be world president for a day I'd introduce international sizing for clothing!). So, here is a rough guide:

* Generally, men's and unisex sizing is by letters (S, M, L, XL) and women's by number (8, 10, 12, 14). European sizing often also uses numbers, where XS=1, S=2, M=3, L=4, etc, and we present it as XS/1, S/2, M/3, etc.
European sizing is generally one size smaller than Australian sizing, for both the top half (jerseys, vests), and the bottom half (knicks and bibs), but the same as Australian sizing for mitts/gloves and socks (see below). There are exceptions, eg, the RadioShack and 2014 Orica kit are made by Craft with Australian/US/UK sizing, ie, one size larger than the usual European sizing. (See what I mean about having international sizing!!)
* It is quite common for guys to be one size larger on the top than the bottom.
Women choosing unisex gear, especially European team kit, should be guided by the chest and waist measurements, but not the hip measurements (eg, in the Santini table below).
* Most women's jerseys are tapered at the waist and flare a little on the hips, and some have bust darts or a contoured bust, but if you don't need the contouring you'll find a much larger range in the men/unisex sections.
* Most women's knicks are wider on the hip measurement, so if you have narrow hips check out the men's range as well.
* It's not uncommon for children's knicks to fit everywhere except the legs always seem to be too wide!
Jerseys, tops, vests & jackets are measured in cm, under-arm seam to under-arm seam, when the garment is laying flat on the table and stretched slightly, so double it to get the garment's chest measurement, and this should be equal to or larger than your chest measurement - how large depends how loose you like to wear it. 
Knicks and bibknick measurements are waist size in cm or inches, and the fit should be a really firm, so go for the skin-tight size, or one size down if you're in between sizes. Waist size is the most important, because it relates to the crutch fit, and you want to avoid any movement in the crutch. If you have larger / thinner thighs than Mr Average, the knick or bibs may be firm / loose at the cuffs, and if you are taller / shorter than Mr A, the bibs may not be an ideal fit on the shoulders, but the crutch will be correct, and that's where it counts! 
Mitts & gloves don't have a measurable size (inches or cm) but sizing is reasonably consistant, ie, XL is XL in most brands. Some manufacturers give a measurement across the knuckles.
* Foot sizing - for shoes, socks and shoe covers - is the most accurate of all (horray!), although there are 3 internationally accepted sizings: European (36, 42, 46, etc), UK/Aus (8, 9.5, 10) and US (8, 9.5, 10), but beware! UK and US sizing are not the same! eg, 44 Euro = 9.5 UK = 10.5 US.

UNO and Santini size tables can be viewed below or downloaded via the links. Note, Nalini sizing is almost the same as Santini. The BMC gear is similar to Santini's sizing; also Adidas.