First, look at what lot sizes you have in your game. What you can use will help you decide how to scale things. I am not including height in my scales, as you can use any wall heights you want on any lot. Look at the smallest one and think 'okay, I must be able to fit all the basics in there and maybe a pool if I get lucky' then look at the biggest one and think 'okay, that one is going to be huge and I will have to fill it with extras', then look at the one you see the most of and think 'okay, that is an average size house.' This may/may not help.
A toilet is one square, but how much of it gets filled depends on the toilet. In real life, toilets tend to be about the same size. So just look at your own toilet and think how big it looks. Two-eight times your toilet's size will probably make a bed, depending on the type of bed. Bare minimum toilet room = 2 squares, 1 toilet and 1 door. If you want a bathroom featuring a toilet, maybe 9 squares is about average. But four square bathrooms are also doable and super fun to make. These examples are to celebrate the first ever item made for The Sims.
(the toilet scale just helps me, okay?)