Your heat mat should take up about 1/3 of the enclosure floorspace, depending on the species you are using it for and the climate you live in. Perhaps 1/4 of the floorspace if you live in a hot area, and perhaps 1/2 of the floor area if you live in a very cold area.
They should be used with a thermostat but sometimes (especially with the very small mats) you can go without, as long as you pay very careful attention to the temps it's holding and how much substrate is covering it.
I have used a variety of heatmats successfully, for smaller exoterra 30x30cm knobtail enclosures I used a 15x15cm 4w mat with good success. Likewise I heated 60x30cm knobtail aquariums with 30x30cm 20w mats.
Be very careful with how much substrate you cover the mats with and take notice of the warnings on the pack. If you put a little bit too much over it, it wont get warm enough, but if you put way too much substrate over it, it will become incredibly hot. When I first started out keeping knobtails I had a 4w mat that managed to max out my thermometer at 70º and climbing. That was with about 5-6cm of sand over it as the geckos had done some evening excavating. Seeing my knobbies do the 'hot sand dance' over it was funny, but it could have seriously injured them and is an excellent reminder about constantly checking temps and using thermostats.
My experience was also a good lesson about taking notice of all warnings and instructions given to you when using electrical heating devices, as even something as harmless as a tiny 4w mat could have killed my geckos and potentially started a fire in my house.
Having said that, I believe these devices are perfectly safe and useful, as long as they are used properly.