As for everything else, it all depends on what the other dog is showing you, and what your own dog is doing.
For instance, in my case: my dogs are a decent size, quite comfortable meeting strange dogs, and are pretty good at handling all sorts of situations without needing my help. When they get the feeling that the dog approaching is a bit sketchy, they tend to just avoid them; they don't look for trouble, they just keep on walking.
So, here’s what I do if an off-lead dog approaches mine while they're on-lead...
- I give them as much slack as possible, or even drop the lead entirely.
- I WARN them that another dog is coming (in a calm voice, mind!), to avoid them being startled and to show them that I’ve got things under control (I usually say, "have you seen the pup?", and if I say that in the middle of a walk, they’ll start looking all over the place to find where the dog is!).
- By default, I let them interact, but as soon as they've had a sniff, I keep walking at a brisk pace and encourage them to follow me. I make a habit of not letting encounters with unknown dogs drag on, especially when the owners don’t look like they know what they’re doing; we say a quick hello and part ways calmly :)
- If I don’t like the look of the other dog, then yes, I try to step in to at least lower the tension (as if to say, "OI, I'm 5'7" so you’d better pack it in if you want to say hello to the little one I’m with"), or even calmly shoo it away. There’s no secret to it—to make it work, you have to be assertive: stand up straight, move towards the other dog, and shout "HEY!" in a deep voice, for example. I did exactly that a few days ago with a Pointer that was starting to pick a fight with my entire male; there was no owner in sight, and he soon calmed down and went on his way. Now, if the other dog is truly determined and aggressive, well... you just have to hope the owner turns up quickly to get their beast back on a lead, because you won't be able to keep that up for long. Basically, other than hoping you never cross paths with a dog that’s genuinely intent on mauling yours, there isn’t much else you can do—when that happens, whether you’ve got a small dog or a big one, you soon find yourself feeling pretty helpless.