Tool - Spirit Level
72 in. Box Frame Level
You should always use the longest spirit level that will fit in the space you have available.
Here's a low cost 6 foot long one that I like...
http://www.harborfreight.com/72-in-box-frame-level-69114.html $19.99
The nice thing about a low cost level is that you will worry less about getting glue and concrete all over it. A low cost one is even the best straight edge to use when screeding wet concrete as you can afford to throw it away afterwards and buy a new one. I typicaly have a couple of these low cost levels on hand.
With any level it is worth occasionally double checking by rotating 180 degrees it to make sure it still shows the same reading. Also check that it agrees with what your self leveling laser is telling you.
.jpg)
If you want the highest street cred on a job site then Stabila is generally regarded as the most accurate brand of spirit level, but they are expensive. Sometimes you can get a bundle deal that includes two different sizes. Here's the pair I brought for about $90:
http://www.amazon.com/gp/product/B000E7SV2I/ref=ox_ya_oh_product .