A method performed by a communication device comprises identifying a long transmission time interval (TTI) transport block size from a set of long TTI transport block sizes used for transport block transmission within a long TTI. The method further comprises determining a short TTI transport block size, used for transport block transmission within a short TTI, by downscaling the long TTI transport block size based on a relation between a number of symbols in the short TTI and a number of symbols in the long TTI that is greater than the number of symbols in the short TTI.