Are they agressive? No, they are territorial (like was said) and this is mostly towards males of their own species (that includes all colisa lalia color morphs!). However, don't assume because they are only truly 'aggressive' towards other males that it's fine to keep them crowded in a small tank with females. Males also chase the females - and that can lead to serious stress for her and could even kill her as a result.
How big do they get? 2"
How big of a tank does one need? Theoreticaly, a 5 gallon would be enough for a single dwarf (without any fishy tank-mates). However, they are sensitive to water quality so at least a 10 gallon for single male OR for a couple of females. Don't keep 2 males in a 10. Also, don't keep a male/female pair in a 10 unless it's very heavily planted. They're fine with tank-mates as long as you don't over-stock.
How big of a tank do two need? If they are males, go for 10 gallons per fish. For females, a 10 gallon is enough for a couple. To mix them, a single male with 2 females is fine in a 20 gallon.
Do they do better in group? It doesn't matter with males but the females appreciate company.