general advice from the makers is dia. = beam of boat. that tends to be a bit mean for monohulls and generous for multis. contact aladdins cave for advice, they stock an australian brand which is quite good value.
You will probably find that you want a 12 footer and it just so happens that the cheapest and best option is available from a mail order army surplus place in the States... http://www.swedeandsons.com/ They do a 12 footer for about $150
the drogue is a drag device you stream from the stern to slow you down. the para anchor is set from the bow to hold you 'still' in the water. there is a place for both; i have only the anchor and would like to get a drogue as well, have you tried the series drogue in anger?
the parachute anchor is just a more or less marinized version of a flier's parachute, it has no rigid structure. not to be confused with the traditional Voss-type sea anchor which has a metal ring to hold the mouth open and is a lot smaller, (now called a drogue).
No.. I didn't try it in bad weather, this is like the liferaft.. just in case.. but I was realy convinced by what I've read on the subject.. (fortunately I never had to sail with winds over force 7/8 )
Try on Google at "Jordan drogue".. plenty to read..
Lyn and Larry Pardey detail effects of a paranchor, and typically 8ft is used on 25-36ft (from memory). The bigger the sea-anchor, the more difficult to deploy and recover, and if your boat is an easily hove-to boat, it only needs the sea-anchor to hold the head up a bit and prevent the boat from sailing out of its slick.
<hr width=100% size=1>Black Sugar - the sweetest of all