How can I use the capacity of a dam as sensitive input?

Unfortunately HOMER can only model run-of-river hydro systems, not ones with storage reservoirs.  So you can't use HOMER to consider the trade-off between electrical storage (in the form of batteries) and hydro storage in the reservoir, nor can you enter the reservoir capacity or do a sensitivity analysis on it.

HOMER accepts either monthly or hourly stream flow data, so you need to do some work on your daily data to use them in HOMER.  The best thing would be to create an hourly data set by assuming that the stream flow remains constant throughout each day.  So you would create an 8760-hour data set from your 365-day data set, where the first 24 hourly values are equal to the first daily value, the second 24 hourly values are equal to the second daily value, and so on.  That requires either writing a program or doing some work in a spreadsheet.