There is no such thing as a standard size in biking.  The best we can do is the size everyone decides to use.  The industry is continually beseiged with new standards which are intended to help.  Sometimes they do but mostly they don't.
Here is a run-down on historic shock sizes (now called "imperial") and the new (metric) shock sizes.

We used to have about 12 imperial shock sizes.  Now with metric we have an additional 5 frame sizes, 10 shock sizes and a huge number of custom strokes inside that.

Historic Imperial Sizes

Historically shock makers would make whatever frame makers wanted.  Eventually common sizes were settled on but never formalised.  Many of these shocks sizes were very tight for internal space.

New Metric Sizes

In the mid 2010's the biggest shock makers all agreed on new frame sizes.  These have longer length for more internal room and feature standard length changes of 20 mm with 10 mm steps in stroke. 
Sounds great right?  But wait.
Now within each frame size they can have travel spacers to reduce stroke in 2.5 mm increments so bike makers can get exactly the travel the market segment desires. 
Also, each frame size and stroke can have two different head styles:
•   Eyelet heads use the same 1/2" DU bushings that imperial shocks used.
•   Trunnion heads make the shock 25 mm shorter and mount with bolts straight into the head of the shock using bearings in the frame.
Bearing options also exist for eyelet shocks.  These are all 30 mm wide and take 8 mm bolts.

Trek Example: A 230 x 57.5 mm shock is an eyelet shock in 230 x 65 mm frame with a 7.5 mm travel spacer inside.
Giant Example: A 185 x 52.5 mm shock is a trunnion shock in 185 x 55 mm frame with a 2.5 mm travel spacer inside.  Same internals as a 210 x 55 mm shock.

*Stroke is reduced from maximum using internal shaft spacers