Good size for a computer monitor?

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What would be a good size for a computer's (MacBook Pro/Apple's "Pro" laptop) monitor? I have been dreaming about this HP 2159m 21.5" monitor (link- http://www.shopping.hp.com/product/display/display/1/storefronts/FV585AA%2523ABA ). I just love it's design, both DVI-D and HDMI inputs and it's internal high-speakers, which are only 2x2W though... Is 2x2W enough for high speakers? I know that it isn't a lot, and that at least 40W would be great but... My question would be: "Is 21.5" enough for gaming (both hard-core and casual), video/audio editing, surfing the web and doing other normal stuff?" I know that you can do all of those things with a tiny screen, but I want to do them comfortably. I am maybe not going to buy a bigger/smaller screen anyway, but I just wanted to know your opinions. At least the MacBook Pro is enough efficient for running it, but I don't know if they work together, but I surely hope so. I have a Mini DisplayPort-DVI adapter, so that I can connect them together. I could also possibly buy an Mini DisplayPort-HDMI adapter and cable, but I don't know if the second option is worth the extra money, even if it's as small as 10€ (Yes, we have euros and not USDs). If anyone would have any suggestions on this also, they would be gladly taken. [b]ATTENTION!!!! HDMI AND DVI HAVE THE SAME VIDEO QUALITY, BUT HDMI HAS ALSO THE AUDIO, UNLIKE THE DVI![/b]