When you look at a map of the US, how was it decided how much area each state should have and lines drawn?

Answer 1

The decisions were political--decided by colonizers like England, by war, by land deals with other countries, and by Congress and the President.
The boundaries were sometimes based on natural features--as previous answerer indicated, or sometimes based arbitrarily on latitude or longitude.
The boundaries of the first thirteen states were influenced by the boundaries of the early colonies.
In addition, some boundaries were decided by war with European countries (or their successors) (the states bordering Mexico and Canada, for instance), or by land purchases (Louisiana, and states along the Mississippi; Alaska).

Answer 2

a lot of state borders are along rivers or other natural landmaks, but in states with staight line borders they generally are just drawn and decided on