I'm making two assumptions:
1) The largest single national group on this board is USA, and the majority of posters on this board are from nations with blood group percentages similar to the USA.
2) The average penis size of guys on this board is bigger than the average penis size of the world.
These are big assumptions, but if there is anything in the idea of a link between blood group and penis size it might show up even on this rough and ready survey.
Using just the ABO system (leaving out rhesus + and -) the USA percentages (according to Wikipedia) are:
O = 44%
A = 42%
B = 10%
AB = 4%
With 65 responses the percentages for the survey are:
O = 37%
A = 28%
B = 20%
AB = 15%
This looks like a statistically significant result. B and especially AB seem to be associated with a large penis, and A not associated with a large penis. O seems not to have an impact one way or the other.
It would be reasonably straightforward to set up some sort of survey taking into account rhesus + or -, penis size, nationality and ethnicity.