Here are most of the fish tank dimensions that you will see. Not all these sizes may be the same depending on the company making the tanks but they are a great resource to get an idea.
LxWxH
5.5g 16x8x10
10g 20x10x12
20high 24x12x16
20long 30x12x12
29g 30x12x18
30g 36x12x16
33long 48x12x12
38g 36x12x20
40long 48x12x16
40gbreeder 36x18x16
55g 48x12x20
60g 48x12x23
75g 48x18x20
90g 48x18x25
110g high 48x18x30
120g 48x24x24
125g 72x18x24
150g 72x18x28
180g 72x24x24
210g 84x24x24