Really depends how much growing space you have - and what you plan to grow.

I have a "Planters Pride" greenhouse kit - basically a tray, clear lid, and a heating pad. I use it to start the peppers and tomatoes and then move them out of the greenhouse.

Keeping the lid on all the time and the heating pad on all the time - I've seen temps get over 100 on occasion.

Here is a link that might be useful: My Garden Blog

I have about 6 or 7 the ones I have are two sizes 48x20 and 20x20. You have to understand that in order for the heating mat to work good and heat it has to be put over a wooden base. Never on concrete or floor. The heat escapes.
They are great they are helping me to grow all kinds of vegetables Also they are helping me to get fruit tree cuttings to root and expand my Orchard.

I used a 20 x 20 Seedling Heat Mat on a wooden table in front of a south-facing window to start my tomatoes, peppers and eggplant in January. Was a little concerned that it would get too hot, but that wasn't a problem and it really helped the seeds germinate more quickly.

I did find that once we started getting more sunny days (towards the end of February) it was necessary to unplug the heat mat since the combination of sun and heat from below dried peat pots very quickly.

My seedling heat mats are the same size as a standard flat. It also very clearly states on the mat that it will heat the soil to 10 - 15 degreef F above the ambient room temperature. In my 60 degree basement it is perfect, but in the 70 degree dining room window, those Tomato seeds were toasted and never germinated.

Steve