Most people have come across a bathroom with a toilet tucked into a corner somewhere that just seems to small for the average person to fit into. There are standard codes for building in place so this does not happen. You need enough space on all sides, not only so you can fit sitting on the toilet but also so that you can get to the plumbing behind the toilet.
Measure from the side wall to side wall where you are going to place the toilet. You will need to have at least a 30-inch opening to pass the building code for installing a toilet. This measurement is meant to give the proper amount of elbow room so if you are working with an angled corner, just make sure the fattest part of the bowl is sitting in a 30-inch space, even though the tank area might be tighter.
Decide how big of a toilet you can install. The standard toilets are larger than they used to be but you can still get the smaller ones if you have it ordered. Typically they are called cottage toilets and are made for homes with small bathrooms. If you are really cramped for space, try using a high tank toilet as pictured.
Measure the area directly in front of the toilet. This needs to be at least 24 inches by code. Even though this might seem to be a lot of space when you are just drawing out your plans, in real life, it gets used up real quickly when a human body is standing in it. If you are limited in space, check to make sure you don't have an elongated toilet (17-inch bowl instead of a standard 15-inch bowl).
Find out how large the old toilet is if you are just replacing a toilet. Measure from the centre of the back mounting bolt to the back of the toilet. Write this measurement down and compare it to a new toilets measurement. If the new one equals the measurement or smaller, it will fit over the drainage pipe already installed.
Check the measurement from the base of the toilet to the top of the seat for the height of the seat. If you have young children, you may consider getting a shorter toilet which is about 14 inches to the top of the bowl, while if you have older adults with weak knees, a higher toilet of 17 inches might be more comfortable.
Install a pedestal sink for more bathroom space.