BTW - how big were the sea mines dropped by parachute as
'blockbusters'?

The Germans used the LMA and LMB Minenbomben:


http://www.luftarchiv.info/bordgerate/abwurf.htm

On this German page is a range of German bombs and you can clearly see
the LMB mine fitted with bomb fins and the last one the BM 1000
"Monika". The Germans also had a LMB IV that was intended for the
Me-264 but I don't think it was ever used as the Me-264 was lost. It
is noteworthy that this weapon would have been used against the US
eastern seaboard by the Me-264.

Rob

p.s. The LMB IV weighed 2,380 lbs with 2,006 lb of Amatol 39.