Time of concentration - Difference between large and small basins
This is a difficult question since time of concentration ($t_c$) can in theory,  vary substantially, for two basins of equal size.

Off the top of my head, I can imagine several other variables that would be necessary to determine $t_c$: drainage density, basin relief, basin length, surface roughness (Mannings coefficient), and rainfall statistics. There's certainly others too. 

I found a paper titled The characteristic time scale for basin hydrological response using radar data is freely available. There is a discussion of $t_c$ beginning on page 93 at bullet point number 2. They cite other papers in their discussion that would be worth tracking down and reading. 

Note they present an empirical equation for $t_c$:

$$t_c=5.4\left(\frac{L}{S^{0.5}}\right)^{0.75}$$

L is basin length, and S is slope/relief. It's weird, yet I'm not surprised since it's empirical. Evidently the 5.4 has dimensions of $time$ and $length^{-0.75}$. It's hard to imagine that this equation would actually be useful.