I remember a recent paper on this topic here. You won't be able to do a power calculation from fastq files, as you need more information about the test you're carrying out. You can check out the RNASeqPower package in bioconductor. Alternatively, there's the pwr package in CRAN, which is a good baseline framework of standard power calculations.
Hi Andrew,
I'll try to use the tools you suggested. Really appreciate it !