Short BSR & Truncated (both are size of 1 Byte) are considered as Padding BSR.
And Long BSR is longer than above.
if Padding Space is only 2 bytes,add short BSR
else if Padding space is between 2 and 4 then add Truncated BSR.
So Your next question should be How Short & Truncated BSR are different?
Short BSR : It is to indicate eNB that data is ONLY available for Single LCG(BSR has LCG ID). i.e UE has no more data available(at RLC & PDCP) to send.
Truncated BSR : Name only suggest , i.e Data is available for MORE THAN ONE LCG but I could request for only one. By this eNB understands that ,UE has still more data to send so that eNB can give more/extra grant next time.
LONG BSR : It can indicate eNB about data availability for all 4 LCGs.
Hope It clears your doubt.