How to get parameter estimation r and p for Negative Binomial distribution using R?

Hi All,

I want to fit read count data to negative binomial distribution and estimate parameters using MLE method to get r=number of failures and p=prob of failures. I tried using fitdist function from fitdistrplus package, but I am unable to understand the estimated parameter size and mu if they are the same as r and p.

Has anyone used this function? Could you explain what does the estimated parameter mean? It is quite confusing to understand. Or suggest another function in R to do the same?

Any help will be much appreciated. Thank you!