what can i do with 5mm glass??

[Reef Puffer]

how big can i go? i do have a tempered base that is 2 1/2'x 2 1/2'x 12mm if i decide to use it. but lots of untempered 5mm glass aswell. this calculator (http://www.theaquatools.com/building-your-aquarium) says if i go no more than 14" high i can like 1 000 000cm X 1 000 000cm or more? just doesnt seem right? but at 16" high i can get almost 4' square? anybody actually used 5mm glass? how high can i go? what are some different dimensions people have done/seen themselves? maybe long and skinny? i just need room for my FW id sharks to swim and if i have to buy a tank its goin salt! help muchly appreciated! thanx