There is no standard size. It all goes by the manufacturer, system design, line set capacity, and other such variables. Blocks on an older system are different than those manufactured today.
8, 10, and 12" diameter blocks are the most common (although there are other sizes), and they are available in both upright and underhung configurations. Multi-line sheaves (the rotating part of the block) are commonly available from 1 to 8 lines (plus the rope groove for head blocks). Some rigging system designers use single line loft blocks with outrigger guide pulleys for the passing lines, while others use a multi-line sheave for each set. The multi-line sheaves are not any more expensive and they tend to reduce the operational noise and friction of a line set. Also, head blocks for double-purchase counterweight carriages (arbors) are slightly different than those for single-purchase arbors. Originally, the sheaves were wooden (when they were used with fiber rope), then cast iron, and now most are nylatron plastic (with the exception of Fire Curtain block sheaves, which must be steel so they don't melt during a fire.The fleet angle (the angle that the rope departs from the pulley groove must be 1 1/2 degrees, or less, so crooked blocks wear away at the ropes and the sheave grooves. Excessive fleet angles also make noise as the cable strands pass over the pulley flanges. The side-loading created by excessive fleet angles also adds undue wear on the bearings as they are primarily designed for forces perpendicular to their axles. Where the loft beams and/or head beams are not level the loft blocks and head blocks need to be mounted so they can pivot and allow the cables to exit the sheave groove(s) without excessive fleet angles. The reason loft beams and head beams may not be level is because the roof of the stage house needs to be slightly sloped to facilitate storm water drainage. If the roof is flat then water pools in-place and can cause leaks over time. Roof leaks can cause significant damage to lighting equipment, drapes, floors, and the rigging equipment.Old wooden sheave blocks and low-load / cheap blocks typically use plain bearings, while modern blocks use either ball bearings or tapered roller bearings (Timken bearings). The preferences for one or the other are everywhere when you talk with people in the industry, however, what doesn't get discussed much is the quality of the bearings used. Cheap rigging equipment tends to use cheap bearings which are low tolerance and rough running, which adds noise and friction to the system, whereas more expensive gear uses higher quality bearings and the smoothness of operations results in low friction and low noise. The quality factor of a bearing is it's ABEC number (https://en.wikipedia.org/wiki/ABEC_scale), and a low number indicates a low tolerance bearing that runs rough and has higher friction, whereas a higher ABEC number indicates a better bearing the higher tolerance parts that runs smoother and introduces less friction into the system.
If you are looking for general information then go to JR Clancy's site.Click on this link: http://www.jrclancy.com/blocks.php - when it loads you can choose Loft or Head block and then choose what you are looking for. Each item has a detail page (dimensions included) and documentation…
This should help get you started.