They simply need to be tall enuff for the desired water level in your O/F. The dual standpipes on our 100 gal are quite short because we're running an built-in external O/F. What you do want to be sure of is that the main tube is 1/4" larger ID than the B/H you're using. Use an adapter at the bottom of the standpipe and pressure-fit it into the B/H (sometimes a few turns of teflon tape will make it tighter if needed).
Make sure you vent the cap on the standpipe as needed to quiet it down. You can add as many holes as needed, and you can add an airline valve to the hole(s) so you can "tune" it. Sometimes adding a bleed valve that T's off from the return line back to the sump also helps to tune the system for "max quiet".
I'd post some pix, but the "improved" site doesn't work at all for me (or Renee/Cranberry), so we no longer even try.