How do I interpret the specs of a weather balloon?
The weight of a balloon determines most of its specifications, though I'm not sure if they are generalized or specific to a manufacturer.  Looking at one supplier we can get a feel for what you need.  For a 1 kg payload it looks like you'd want a minimum of a 1200g balloon.  According to the site linked, this 1200g balloon will have:


6.0 ft diameter at launch
113 ft$^3$ volume at release
320 fpm ascent rate
28 ft diameter at burst
109,000 ft altitude at burst
1190 g free lift 


If you need 2kg of payload you'd need to size a bigger balloon.  In general your application is not different from the specs of a normal upper-air sounding launch, so you should be able to spec your balloons directly from suppliers of sounding balloons.  All of the balloons for a 1-2 kg payload should burst above 100,000 feet so it just a matter of picking a supplier and a balloon sized for your payload (per the manufacturer specifications).