What criteria should I be looking at when buying a new motorcycle?

There are a number of criteria you should be looking at:
- Price: How does it fit in your budget?
- Engine type: Do you want a traditional combustion engine, or go for an electric one?
- Number of cylinders: If you go for combustion engine, how many cylinders do you choose? Less cylinders are usually cheaper, make the motorcycle more lightweight and thus easier to maneuver. More cylinders make the engine feel smoother, also have more power, but are heavier and more expensive.
- Usage: What do you use it for? Just for fun, commuting to work, general transportation? Travel? Offroad? Racing?
- Assistance systems: Modern motorcycles have a range of assistance systems like ABS, Cruise Control, Distance sensors, electronic driving modes for different weather conditions etc. They can make driving safer , but also add weight and make the bike more expensive. Which ones do you need?  
- Style: There are different styles like naked bike, cruiser, chopper, dual sport, race bike , scrambler etc. Different styles can be more or less adequate depending on your usage, or you can simply pick the one that appeals to you.
- Size: The two most important aspects in terms of size are seat height and weight. Both have a huge impact on how well you are in control when not driving, for example when you need to stop at a traffic light or going slowly in stop & go traffic.
Seat height is important , so your feet can reach the ground when stopping. However the width of the seat and general width dimensions of the bike also play a role here. So two bikes with same seat height might feel very differently. Best is to try it out