If you're making a soup, then frozen spinach is one of the few things that's as good as fresh spinach. Frozen broccoli should be okay, too, for a soup. I'd say you can put them straight into the pot, frozen, and you should be fine. They will defrost with the heat of the stock. You could microwave them for a minute before hand, just to break up any clumps, if you prefer. I've seen frozen spinach turn into a green brick if it's been in there for too long.
It is compressed when frozen...I'd start with one cylinder and start stirring... it should break apart pretty quickly and you can judge if it looks like it is enough..I can't help you with translating the weights from fresh to frozen, I don't cook that way..good luck..It's packages as cylinders shapes rather than cubes. I'm trying to work out if I should use the same for frozen as fresh by weight.
When I follow a recipe it makes about 6 portions. Since I live alone, I want to try and get it right first time.