First you have to decide the step and size/rows of the RRA's. To calculate the step and the row count you have first to define the periodes you like to store and the maximal chart width and the rrd interval. Use the calculated values to pick natural values.
STEP = [Period in Seconds] / [Interval] / [Maximal Chart Width] ROWS = [Period in Seconds] / [Interval] / [STEP] * [Period to keep]
1 Day period, 600px width, 60sec interval, Store 1 period.
STEP = 86400 / 60 / 600 ⇒ 2.4
Use 2 as step size to store one value each second minute.
ROWS = 86400 / 60 / 2 * 1 ⇒ 720