The place where you should start building arena is above dungeon entrance. You should start with building around 200-300 platforms (If You are struggling with some bosses You can also build another platform like this around 20 blocks above first). Main Pre-hardmode fights on that arena are: Skeletron and Slime God. However, you can fight most pre-hm bosses there, but you will have to force biome to summon some of them.
To fight WoF, You should build long bridge, like on vanilla fight, if the fight is too hard for you, put verical walls 3-5 blocks above bridge, with around 50 blocks between them.
At the beggining of hardmode make house for drunk princess and buy candles from her (candles are optional but they help a lot), place them on your arena with around 100 blocks between them, you can place campfires and heart lanters next to the candles. Also extend your arena to around 700-1000 platforms length, make second platform around 30 tiles above first, if you haven't made it before. With that arena and enough preparation you should defeat most hardmode bosses. At hardmode you can easily force biomes on arena by buying fountains from Shaman npc, so you can fight most of the bosses there. Only hardmode bosses outside the arena will be: Plantera, Aquatic Scourge and Golem. For plantera and golem you should make similiar arena as in vanilla gameplay (Arena for Plantera should be a little bigger though). For Aquatic Scourge just use either Water walking boots (Angel treads) or make a platform just above the water.
At post-Moon Lord Your arena should be extended to 1500-2000 platform length, and you should add 3rd platform above. On post-Moon Lord there are more bosses that you can't fight on that arena: Polterghast, Signus and Yharon. (Ceaseless Void will be easier on arena than in dungeon, even though it enrages).
For Polterghaist you should expand dungeon by placing unsafe dungeon walls, you can make one platform in the middle.
For Signus you can use WoF arena, after removing walls above bridge.
For Yharon You have to make box in jungle with around 115 blocks in both directions from the middle, you can make platforms on all the length, with around 100 block between them in heigth.
For Supreme Calamitas make a single 100 block length platform, and summon her being in the middle of the platform.
IMPORTANT NOTE: You actually need Fargo's Mutant Mod to force biomes using fountains, I didn't know that while making this guide.