Network coding is designed for wireless networks to improve reliability and throughput. In practical wireless network coding schemes, information is always divided into blocks, and the processing of network coding is carried out within blocks. Size is always the crucial issue during the transmission time of blocks. In the current network coding schemes, the size of a block has to be decided before transmission. If the predecided size is too big, block delay may become intolerable. In this paper, the object function of a block size estimation algorithm is firstly introduced, and then, an adaptive algorithm of block size choice for delaysensitive applications is designed. And the tradeoff between block delay and throughput is discussed. By adjusting the sizes of blocks in wireless networks adaptively, this algorithm can satisfy the block delay requirements of the applications when the network coding scheme is running.