RNASeq: Normalization, stabilization, gene length and rlog
I am not sure what you mean by "decimal data", however, if you really want to use "gene length" information for normalization, take a look at EDASeq. Unlike DESeq or EdgeR, its normalization step takes gene length into account to produce "normalized counts", which you can feed into DESeq or EdgeR according to their manual.

Below is from the said manual: 


  The normalized counts (or the original counts and the offset) obtained
  using the EDASeq package can be supplied to packages such as edgeR
  (Robinson, McCarthy, and Smyth 2010) or DESeq (Anders and Huber 2010)
  to find differentially expressed genes.