I am not using 10mm but my understanding is that folks are using the 15mm standard with more figures. One of the attractions of 10mm to me is they cost less. Having more figures kind of negates this.
I am doing 40mm X 20mm for 15mm figures but only with one rank of figures. Latest posts have me wondering if I shouldn't go a bit deeper? Anyway, as I don't have a huge amount of painted figures, I won't be going with double ranks of figures. I have debated basing and leaving space for future ranks of figures or going with even shallower bases and double them up in the future. I am still in painting mode so I still have time to decide.
Others in my area have 40mmX20mm basing for their 15mm figures. I am in San Antonio and the Austin group is going this route too. The other challenge we have is that many have either Shako/Napoleon's Battles basing or Empire. Shako can easily be doubled up but you have the issue of whether the player has enough figures for doubling. The Empire basing requires Sabot basing. I do like the look of double ranking but dread the extra painting right now.
What brand of figures are you going with in 10mm? If Old Glory, then the 5 wide would make sense and it is up to you about double ranks or not. The other benefit of 40mm wide is that you can play against someone with 15mm figures as the basing frontage is probably most important.
Chris