My pasta pot/steamer did not come with directions telling me how full to make it. It seems to boil over or not have enough water to cover the pasta. It is a large pot with the pasta strainer included.
The pot choice should depend on the amount of water (which depends on the amount of pasta you're cooking) rather than the other way around. For detailed advice on amounts, see these answers (answer 1, answer 2).
Generally (not using fresh pasta or very long pasta when more water is necessary) you just need to cover the pasta. A bit of oil on top will help keep it from boiling over, as will keeping the lid off after it's begun boiling and just simmering it rather than a rolling boil.
With longer pastas like spaghetti you should add more water than just enough to cover the pasta, but as long as the top of the water is more than an inch below the top of the pot (after adding the pasta) you shouldn't have trouble if you follow the precautions above.
There's two schools of thought to this. I usually roughly half fill the pot with water bring it to the boil, and add pasta to make it roughly 3/4 the way up (I always use the same pot), a splash or two of olive oil (well more than that. The perfect pasta for me smells nice and strongly of olive oil, and edible with nothing else should i choose to, so i add a generous quantity) and salt. I bring it to a boil, and turn down the heat, and keep stirring it. I never had issues with it boiling over.
You can however also start with cold water add the pasta then bring it to the boil. You should use a lot less water in that case. I'm still working on this since i've had perfect results with the first method, and am still breaking the mental barrier to too little salt.
You specifically mention a "pasta pot/steamer", so I'm going to assume that it's one of those pots with a strainer insert that stays in the pot while you're cooking the pasta. (and then you lift it out to drain the pasta).
The problem with most of them is that they're designed for steaming, and so the insert stops an inch or two below the level of the strainer portion. This means you need more water than you'd need for the pasta in a standard pot; it's only really an advantage if you're making more than one batch, as you can lift it out, dump it, then add more pasta (or vegetables) to already hot water.
The second design flaw in every one that I've seen is that the strainer insert continues a couple of inches past the top of the outer pot. This is great if you're steaming ... but it's a problem because it makes it more difficult to tell where the outer pot ends when you're looking at the inside. You need the water level to stop 3 or more inches below the edge of the outer pot ... if you don't, when you add the pasta and bring it back to a boil, the water flows out of the pot and onto your stove. (and being starchy pasta water, leaves this film you have to scrub off as it instantly boils off).
So ... my advice would be:
- fill the pot of water without the insert, and stop about 4 inches (10cm) from the top of the pot. Then insert the strainer if you want to use it. Adjust the distance for bulkier foods that will displace more water (eg, blanching vegetables).
- consider not using the strainer in the pot when it's boiling.
- consider turning down the heat when the water resumes its boil after adding the pasta, so that it's at a boil, but not overly vigorous (to make it less likely to spill out that seam between the strainer and pot).
And for those considering buying one of these -- if you're going to be using it almost exclusively for boiling and rarely for steaming, look for a strainer that gets close to the bottom of the pot.