Starting at higher levels
When starting at higher levels, measure starting cash in gold instead of silver, and multiply by level; a level 3 starting character has 3d6 (x30) starting gold, or 90 to 540 gp. Roll for 1 magical treasure per level; a level 5 character would roll for 5 random items as if from a level 5 monster. The character can then buy and sell as needed from there.
What impact does size have on the game?
Size impacts hit dice (see the top of page 27, first bullet) and number of creatures you can attack at a time (see page 18 under range and distance). The bigger you are the more HD you have, and the smaller the fewer. You can engage in melee with 4 creatures of the same size; I would subtract 2 for every category smaller, and add 2 for every category bigger; a wyvern is huge (2 categories larger than humans) so up to 8 humans can melee against a wyvern at once. A rat is small, so only 2 humans can attack a single rat at the same time with melee attacks.
Gnomes and stoutlings are medium (but at the very, very low end of medium). I toyed with having them be small and giving them D4 hit dice, but that made them much less playable. So I tweaked the sizes a little. Wood trolls are large (hence the D8 hit die). A few monsters have abilities that vary by size ('can swallow a creature medium-sized or smaller on a critical attack'), but that's about it.
If you want more variety in your smaller folk and their mechanics, it's an easy plug in:
New Talent: Little Folk. You have learned to leverage your smaller size in combat with larger foes. Any foe larger than medium size takes -1 to attack rolls against you.
What impact does armor have on spell casters? Are there penalties for wearing armor?
There is no relationship between armor and spell casting. As a magic user, you haven't spent any time or energy learning how to wear armor or carry a shield, because it's just not your priority - not because of rules. Specific bonuses/penalties for armor/spell casting rules were a little to crunchy for my taste for where the game is.