Comparing different large chainring sizes for the same cassette?
Mike Sherman's Gear Calculator can help.

Load your typical cassette settings, then set the chainrings to 34, 50 and 53.
Scroll down to the Speed at X RPM section. You can change the rpms to see how the different chainrings would affect your speed at that cadence. Both the 50 and 53 speeds will show in the chart.

I have a 50-13 as my biggest gear ( a 13-26 cassette) and I spin out at 32-33 mph (about 120 rpm). Any faster on a downhill, and I'll just coast.