I don't like different sizes for bank notes. I'm okay with lack of color variation with money, though I guess we can make money the color of the rainbow.
I'm gonna guess you're American? I feel like this is one of those things that you just don't see the advantages of because you've gotten used to what you already have. Like, when would it be inconvenient to have different sizes for bank notes? When you're stacking them together I suppose. Why would you be stacking notes of different denominations together? It just doesn't seem like something that happens; even in a bank they're kept separate, and in your wallet they're probably just stuffed in a pocket together, not laid out neatly. It's not like you're likely to have more than a few notes on you at one time after... oh, wait, the US also uses a note for $1 doesn't it? Yeah... still probably not an issue, but maybe you'll have more than a couple of notes at once.
Just coming late to the party, but wanted to chime in as an American who has lived abroad a lot.
I have used the US dollar, the Euro, the Lira, the Yen, the Rupee (Mauritius), and the Aussie dollar (among others) as my main currency at one time or another. I can say, after many years of experience, that I strongly prefer bills of the exact same size. I do like different colors for different denominations, though.
The reason I prefer single-size money is that I prefer a small wallet. Big bills, even Australian 50s, are a pain to fit into a small wallet. USD always fits just fine. If you are going to change the size of your money, though, do it like the Japanese -- yen vary only in length, not height, and only by a few mm. There are no bills the size of paperback pages (Euro) that won't fit in a purse, let alone a wallet. When the height of a bill doesn't change, you can use the same size wallet for all the bills, even if you have to fold one end. To make a big Euro note fit in a small wallet, it's two folds (lengthwise and vertical) to make it fit, and that's horrific.
And really, if you can't pay attention to the gigantic number on the bill, it's your own fault. (Unless, as mentioned previously, you can't see. I think braille is the way to go there, or notches, or something. We definitely should do something about that. I remember taking money as a cashier and most blind customers folded each bill a different way.)
I will say, we need to retire the penny and just round up/down as needed.
