I'm thinking of upgrading the car battery any recommendations? it will need to be heavy duty as not only has the vehicle got stop-start but also running a small sub.  combined with short trips the current efb battery is having to be recharged every month near enough. I was thinking of the Bosch s6 as it is supposed to have a faster recharge . also what is the max size I can fit into the current battery holder (l x w x h)

i think you need to check the drain on your battery first as you could be wasting your money . it could be that the mods  (sub -amp ect ) may be draining the battery when car is standing .this drain needs checking first and isolating if to large a draw .. no matter how large a battery. If you don't check for this you will just ruin a new battery of any size ..  

if you are having to recharge it then you are taking more out of it than the alternator puts in.  Putting a bigger battery in it does not fix that. Unless you are saying periodically you do a much longer drive that fully charges the battery and the bigger battery might have enough reserve until you do your next long drive that fully charges the battery.

Ford say that for stop/start you must use identical spec battery. I do not know how crucial that is. also I don't know what is expected to go wrong if you go against their advice. If the worst is that the engine won't stop at traffic lights then I could live with that. I disable the stop start when I get in the car - if I remember to.

If you instal a larger battery then make sure the battery leads can reach. I would have thought they would fit any battery that would fit in the battery carrier as long as you get one with the terminals at correct corners (they do vary), but might be worth checking for sure.

Putting a bigger battery in just delays the recharge cycle.

Short trips are your problem.

spoke to someone at work today who advised to check the battery connections as a poor connection could slow the charge rate as well as cause other problems. having checked it this evening all the connections are tight but the ground point has been painted at the factory so it's only being grounded through the bolt thread. i was under the impression the ground point should be bare metal for best performance is this correct?

I'm thinking a larger capacity AGM battery will require charging less often if at all for 2 reasons. 1 it accepts a faster charge  so will be charged more on each trip and a larger capacity will take longer to empty any faults in my thinking

as agm is an upgrade to efb it should work without any issue