Size matters a lot, however if you have good checking stats you can still hit hard even if you're small. The advantage to being small is being more agile. so as a D-man you probably want to be taller, but if you enjoy being yourself in the game you can still check as a small guy.
The checking ability is THE driving force behind checking. last year strength played a huge role in checking, however this year i don't notice it playing nearly as big of a role. Aggressiveness doesn't actually increase your ability to check, rather it increasing your ability to get huge hits(i think, but im not sure) but the main thing aggressiveness does is increase how much it hurts the other players ratings when you hit them. What i mean if you check one guy like 5 times in a game, he's gonna get tired and be less effective at shooting, skating, etc. If your aggressiveness is higher, he will be affected more.
Basically the checking attribute is going to lead to getting the MOST hits, however if you want to mess the other players up a little bit, add some aggressiveness. As for strength, im on the fence b/c i know last year it was huge, but im not seeing an effect this year. The best thing to do is add checking until it gets pretty expensive, then add a little bit of aggressiveness(like till it costs 20 or 30 points). as for strength, if you want to be safe and take the hit in your other athletic attributes then add some.