crazycatgal I am absolutely not defending those other dogs/owners but in the spirit of giving you practical advice...
Firstly, even though it is v hard you have to adjust your own reaction. Dogs are perceptive and you being afraid or worried is very likely to transfer to your dog and leave him less able to cope with the scenario. Even when you don't feel it, faking a bright, breezy and in control manner can go a long way.
For whatever lucky reason, our JRT (who was attacked many years ago) is a dog fully able to give a quiet, confident, haughty nose in the air to any over friendly dog. It works wonders to put them off persisting with play. It can be easy to think small = vulnerable but often that is not the case, so be careful not to over protect your dog because of his size. I understand that a large dog can over power a smaller dog if it really wants to, but they very rarely want to and little dogs have more in their favour than it might appear.
Picking up a smaller dog can be dangerous. For whatever reason, it can trigger a reaction in the larger dog that leaves him jumping up at you to get to the smaller one. Please only ever do so with great caution and, if possible, with one you holding the larger dog back (obs don't do this if you fear being bitten).
Teaching the smaller dog a "behind me" cue and then using your own body to block the larger dog can help in some scenarios. Body blocking is something dogs tend to understand.
Being assertive with the owners can also help. I have used "are you going to get this bloody dog under control?" before now. It earned me a mouth load in return but I've never seen that dog, off lead bounding all over the street again.
When all else fails, yelling "my dog has an infectious disease" would normally get the other owner running pretty quickly to get theirs back under control.
Finally, even though you shouldn't have to etc. etc. if your country park is filled with off lead dogs and you are not comfortable letting them approach yours then maybe try and find somewhere quieter to walk. I appreciate it is not ideal, not fair etc but you are never going to change all those others and so might enjoy quieter walks better?
Again, none of the above is meant to suggest it is your fault or that it is fair that you carry the responsibility of keeping the peace. It's meant in the spirit of trying to help so I hope it comes across like that.