The key to farming is stability and and setup. A farm with a small setup will more likely be less productive, as larger farms spawn more creatures. Yet, as larger farms spawn more creatures, you want it not to be too large. This is because of the higher chance of monsters dying before they reach the point of gathering. So, i recommend a farm about 120-160 blocks wide, 50-65 blocks high. There are many types of farms but a dart/spear trap is almost the highest in production and effectiveness.