The purpose of doing cardio when trying to get bigger and stronger is to keep you lean, improve your insulin sensitivity and allow you to eat more calories. What kind of cardio will have the least negative effect on your size and strength gains? That's a no brainer; walking. The great thing about walking is it will not impede your progress in the least, the bad thing is that you have to walk for a bare minimum of 45 minutes to really burn a decent amount of calories and you will not elevate your metabolism much after walking. That is the great thing about interval training; it elevates your metabolism dramatically for long after you have finished your workout.
With that being said, I would still choose a good fast paced hour long walk on the beach over sitting on a stationary bike inside while pounding away on some brutal intervals. I despise intervals on a bike with a passion. My ass goes numb, my balls go numb and I get a splitting headache. Not to mention that I am bored to tears within three minutes. Plus, I think we all do more than enough sitting and the last thing I want to do is sit some more while I'm supposedly "working out;" my hip flexors are tight enough already, thanks.
If you love to ride a bike then that is fine and you should do what you love. But for God's sake, go outside and do it. People will actually get in their cars and drive ten minutes across town to walk inside a gym and sit on a stationary bike and ride it for twenty minutes and then drive home. Why not just ride your bike across town? I don't get it.
So we have established that low intensity, long duration cardio (walking) is the best option for those who are concerned with any losses in size and strength whatsoever. This method was a favorite of many great bodybuilders such as Dorian Yates. Another option here, is to just go for a long slow/low intensity bike ride. Either one works great; but again I warn you to protect your nuts, aka get a good seat.
Next on the list is medium/moderate/high intensity steady state cardio. This kind of cardio is a little tricky because it can elevate cortisol and lead to losses in size and strength. To prevent this you need to be sure to limit the time spent doing this to 30 minutes, max. Two days a week should be safe and three days would probably be ok for most people as well. It's when you get into the 4-7 day per week, 45-60 minute marathon sessions that you see people at the gym doing (whose bodies never change in the least from one year to the next)all the time, that you get into trouble. If you limit your use of this method you should be ok.