How large a rear cassette can my bike take?

I'm just switching my 53-39/25-11 for a 50-34/32-11, which requires a DT capacity of 37 - the maximum allowable with my GS6800 derailleur, but I wonder whether DT capcity is an ideal measure as you would never ride on 34/11 - infact I doubt I'd ever use anything higher than 34/25 - so well within the DT capacity; unless I've misunderstood something. 

For info, I have Campagnolo groupsets on my other bikes and have no issue running 53-39/29-12 on a SC derailleur.

A medium cage Shimano will take up to a 36 cassette but you will need to fit a longer b screw

Chainstay length also affects. The longer, the more tolerant it is of a wider gear range / small capacity RD. Calculators be dammed, i think you just have to use trial and error!

Take a look at this Wolf Tooth RD hanger extender. Claims it can make a medium cage handle 11-36 and 11-40 cassettes.

https://www.wolftoothcomponents.com/products/roadlink

Here's a very useful page wih info about drivetrain capacity:

https://guides.wiggle.co.uk/rear-derailleurs-buying-guide

[difference between tooth numbers on smallest and largest chainring] + [difference between tooth numbers on smallest and largest sprocket on cassette] = drivetrain capacity

We would need to know the current rear derailleur's make and model, and if it's a short cage, medium cage or long cage.  Sometimes just make and model alone will help us to tell you if that particular model only came in one cage size.  There is a method of measuring the cage in MM which can tell us, simply measure the center of axle, of the top jockey wheel (pulley) to the center of axle of the bottom jockey wheel (pulley).  If the measurement is 50 mm it's a short, if 74 it's a medium, if 86 it's a long.  Unless you have a triple I doubt you have a long cage.