Lots of beginners here seem to want to start out on a deadly serious sport bike--GSX-R, CBR, R1/R6, ZX, etc. These bikes are beautiful and sexy, and they really are marvels of engineering, but they are not for beginners. They are twitchy like thoroughbred racehorses, not forgiving. They should be seen as something to work up to.
For a first bike you want something more general-purpose. The same manufacturers make bikes with the same engines but tuned differently for a wider powerband, softer clutch, more suspension travel, more comfortable seat, more comfortable riding position, more reasonable steering geometry. But still plenty of power and handling to make them fun. You're not going to be racing for a while. 8^)
Also you don't want to go out and buy a shiny new bike. You're going to be hard on your first bike. You're going to abuse the clutch. You might drop it once or twice before you get the hang of it. You want something 5-10 years old, perhaps with a few scuffs and bruises on it already. After 6 mos. or a year You can sell this bike for about what you paid for it, and by then you'll have a better idea of what you really want.
I would say 750cc is not too big for a first bike. Bigger for a cruiser, since they carry their weight lower and so they're easier to manage size for size. 500cc is a good compromise. 250 would be very easy to learn on, nice for all-around riding, but if you plan to go long distances at high speeds a 500 is better. A 250 will do 70 mph pretty easily, but running 70 mph all day, it's beating its little heart out. And at high speed it's not as comfortable or as safe as a bigger bike. A 500 is not all that much bigger, physically.
All the Japanese mfgrs are pretty good. They really make a good product--reliable, long-lived, economical, well-engineered, good value for money. I'm a Honda man myself and I think they do a better job, but I'm prejudiced. 8^)
I personally don't like chain drive. The chain is always too tight or too lose and has to be adjusted. You have to lubricate it and the lube flies off and makes a mess and then you have to lube it again. And you have to replace both chain and sprockets every 10,000-20,000 mi. Belt is nice, shaft is better. But chain is lighter and initially cheaper, and a lot of riders don't mind keeping up with it. Maintenance is not mysterious or difficult, just a bother. One guy I know says he just sprays the chain down completely with WD-40 every time he stops for gas. My last chain-drive bike was before WD-40 came on the market (and in those days you had to watch out for those deep dinosaur footprints!)
You want to be able to sit on a bike and put both feet flat on the ground. VERY important, especially for newbies. For people shorter than about 5'10" or so, this can be a major consideration. For really short people (girls), this will severely limit their choice. Cruisers have the lowest seats. And while seat heights are published in the specs, the only way you can really tell is by actually sitting on the bike.
Seating position, the spatial relationship between seat, handlebars and footpegs, is different on every bike. It's mostly a matter of personal taste. Sport bikes tend to have you leaning forward, and too far forward puts a lot of pressure on your wrists, and it's hard on your back too. Cruisers have you sitting straight up with your feet out in front of you, and some people love that but I don't. I like to sit straight up, though. In the end it depends on your preferences, also the length of your torso, your arms and legs. If you sit on a lot of bikes you notice they all feel a little different.