Ok... after some breif research I have the following platform lengths (in metres, listing the maximum length train that can use said platforms)
Organised by region we have:
Western
Dilton Marsh 15
Gilfach Fargoed 16
East Anglia
Berney Arms 18
London North Eastern
Havenhouse 34 48
Midlands
Llandanwg 23
Llandecwyn 22
Tygwyn 22
North West
Birkenhead North Platform 1 - 11 (125,128 others)
Aspartria 35 36 Additioal 137.8, 113.3 unlined
Llanfairpwll 36
Scotland
Duncraig 37
Attadale 40
Corrour 41
Southern
Battersea Pier Staff Halt 3
Durnsford Rd Staff Halt - Up Fast 27
Three Oaks 31
So the shortest mainline platform I could find would be Battersea Pier Staff Halt, with a length of 3m. The shortest passenger platform appears to be Birkenhead North, platform 1 with a length of 11m, and the shortest station appears to be Dilton Marsh at 15m
Ian