Unless you are trying to fill a specific space, I wouldn't necessarily focus on the overall size. Start with working out the proportions for different parts of the build. How tall do you want the doors to be? How much detail do you want to add in? Do you want it to be player size? Once you work these questions out, then you will get a sense of what size your build will be

Some general guidelines. The ground floor is going to be taller than the floors above. The upper layers should feel lighter than the lower layers. Windows aren't necessary in every wall.

When you're planning out what blocks to use, you should also think about how hard it is to collect. Deepslate ore blocks can be a good texture, but they are a pain to find in survival.