@SquirrelTail15, I really can't tell you an exact way to make a floor plan, but I can say if it's a two-story build, always start with placing stairs first and then build your walls around that.
I find that less square or rectangular houses are easier to plan. When there are pieces jutting off from the main structure it helps indicate where a room should be.
As for room sizes, I usually don't make my bathrooms bigger than 3x4, and usually that's in mansions. My standard bathrooms are usually 2x3 and you can fit everything in it without move objects. When I do bedrooms I usually keep them around 4x5 or 5x6, depending on the size of the house. A lot of my floor plans come from hours of trial and error, to be honest. It just takes lots of practice.
I hope this helps. Without an example of what you're trying to go for, I don't really know what else to tell you. There are a bunch of much better builders out there than I am that could help you more.