There's basically two types of dirt bikes: recreational and competition.
The bikes in the 'recreational' category are cheaper, do not have many adjustments or top components but are durable, require little maintenance and are easier to ride.
The bikes in the competition category are more expensive, more powerful, use top components, offer many adjustments. On the down side, they require expesive and extensive maintenance and are hard to ride for beginners.
Between competition bikes, you will find generally motocross oriented bikes (SX or YZ) and woods (XC or WR). The differences may be in the FI mapping, gearing, electric start etc. You can certainly adapt an SX for the woods, but it does require some changes - the smallest change would be the sprockets maybe.
Each of us is different, in terms of height, weight, phisical condition, biking experience etc, so it's hard to give you guidelines. I will instead tell you my story.
I am 5'7"" tall and weigh 65Kg.
I started riding off-road on a 2004 Honda CRF 150F. It is a very nice bike for beginners and very competent for everything I wanted to do with it. You won't go fast, but can successfuly complete a hare-scramble. This kind of bikes make learning easy and are highly recommended.
After two seasons, I moved 'up' to a 2003 Yamaha WR 250F, which I had to lower to stay out of hospital :). This is an awesome bike that would be sufficient to most non-racers. If you're heavy, you could look at the 450 version.
The trouble with it was that I lowered it with a link which messed up the handling. In stock form, it is ok but too tall for me. If I had it lowered properly (more expensive option to replace some suspension parts) it may have been ok.
After a while, mostly out of boredom, I moved yet again to a Husaberg 390. Now this is a different beast. It's a pure-breed enduro racer. Lots of power but smooth. Insanely tall for me but I fear the stumps.
While still inlove with the Berg, I am lately contemplating moving 'down' to a KTM 150 XC. These are much lighter 2 stroke bikes.
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