In real life, dynamite will blow up if you so much as look at it funny. 1 point of damage should always be enough for dynamite of any size.
I've been giving this a lot of thought since the question was first raised. At first I thought there should be some difference between explosives that are "armed" and ones that aren't (dynamite is always armed, for instance, while complex nuclear warheads or electric-charge-activated plastic explosives might not be). But all of the rules I thought of in that regard got too complicated, too fast. So my thinking now is that if the battle is underway, then all explosives are armed, and if you don't want somebody shooting your explosives then launch all your explosives before the enemy gets close enough to snipe at them. (Or don't, because this is BrikWars.)
The next question, then, is how much damage does it take to set off an explosive? The answer is based on their small physical Size, which also makes them very hard to hit most of the time. (Remember that physical Size is different than Explosive Size - a Size 3x Explosive may only be a Size 1" object, because of the different way that explosives are measured.)
Most explosives are detachable objects (for obvious reasons), rather than being a part of a Creation they might be mounted on, so their Structure Level is limited by their own Size. That is to say, an explosive of Size 0" has SL 0, while an explosive of Size 3" has a maximum SL:3. Otherwise you just go by the materials chart in 7.1; very few explosives will be housed in anything sturdier than a sheet metal casing (SL:1, armor 1d10).