So we're more or less starting from scratch - that's fine. But still, it's good you've at least met that stumbling block and are not coming here totally inexperienced in writing (or attempting) battles. Your experience also reflects the opposite problem in itself: A battle can be too short to feel satisfying, but a battle too long (even if it's grand in scale) grows dull.
Keep in mind first and foremost that battles are a form of conflict, and conflict is what drives narrative. Your character is fighting for a reason, and if it's a heated battle, chances are they feel passionately about what they're fighting for. A battle can be used to fulfil the same purpose as a debate or an investigation, however its impact (while possibly overall remaining the same) can be much more powerful and immediate because there is the element of physical risk and danger. The possible reward for victory and price of defeat in battle can be very high, and that's exactly why it's important to give battles a good effort and (so long as they serve a narrative purpose) treat them as an important element of the plot.
Many battles that wind up being short are cut-and-dry affairs: The battle-related posts (or sections thereof) are only a few sentences long, and only describe the action of a character in its most basic form. "[Character X] lunged at his opponent." Is short and very boring to read, and it doesn't give any weight or importance to that action or the fight as a whole. There is so much more to an attack than just naming it: muscles need to move, a character must steel their resolve to slay their opponent (ideally) in a single motion and aim their strike the appropriate location. Adding weight to actions is also an artificial way to lengthen a battle: in-roleplay, the combat lasts just as long, but the post is lengthier, and the battle can benefit from it.
At the same time as their is just what happens in the action itself, there is also the state of the fighters. A man having fought for ten straight minutes, blades flicking about only slightly slower than the eye can follow, will not be in the same physical state as when he entered the battle, and I'm not just talking about injuries. His muscles will begin to ache, sweat will bead his brow, his blood will be pumping with adrenaline. Meanwhile, there is also the relationship between battlers and their environment. How do they move through it as they make this parry or that dodge? Just because you and another fighter are engaged in the empty centre of an arena does not mean the rest of it cannot be used. A good battler (be they fighting to win or entertain) will make use of everything they're presented with. If your opponent is lunging towards you, is it possibly you could dodge out of the way and push him into a wall? Thinking of ways to manipulate the environment is an excellent way to make an interesting battle, and another technique to 'artificially' extend it.
So far I've talked about the content of a single post, and not a battle as a whole. Would you like for me to make a battle thread and I can guide you through the process and offer analysis at each stage?
