To calculate the rad sizes, first you need to know the required heat output for each room, which can be found using an online calculator if you don't have any other way.
Once you have the required output, get a radiator brochure, here's one to get you going:
http://www.stelrad.com/UK/docs/compact.pdf
Scroll down to the page where outputs are quoted. This is where it gets a bit involved. These outputs are for a mean water temperature of 50ºC above room temperature, IE a 75ºC flow and a 65ºC return, with a room temperature of 20ºC. You need a mean water temperature of 30ºC above room temp (half way between 60 and 40 is 50, less the room temp of 20 = 30), so in order to find the correct radiator size you need to multiply the quoted outputs by 0.515.
To make this simpler I would simply choose a radiator off the original chart that is quoting twice the output required for each room, that'll give you a bit in hand that may be useful if it gets really really cold, as CharteredEngineer has suggested.
So, for example, if you have a room that requires 1000w to heat it, find a radiator on the chart that has a quoted output of about 2000w and you'll have a radiator the right size for the room using a 60ºC flow & 40ºC return. You'll probably find that the K2 double panel double convector radiators are generally what you'll be installing unless you have plenty of wall space for a wider P+, and it might be worth looking at rads that are 700mm high as well.
If you want to go down to a 50/30 F&R then the multiplication factor is 0.304, so roughly speaking a room requiring 1000w would now need a radiator quoted at 3000w on the chart. You'd end up with a very efficient system this way but the trade-off is monster-sized radiators, or more smaller ones.
Something else to look at, if you don't already have it, is Vaillant's VRC430 Weather Compensation system, this will also help to improve overall efficiency.
Hope this helps!