bcr229s post is really useful. Those care sheets talk about how to correctly set up an enclosure and handle your snake. Pretty much every health concern (including eating) has roots in proper husbandry. Though your snake is eating, if not regularly so that is a good sign.

The rule of thumb of food items is that the item is as big around as the snake. This can be off a little (like in the cases of two foot long 86g BPs) if your snake is not eating well already but like a said its a rule of thumb, a good guideline, and discussed in the care sheets.

There is frankly no way to tell how old it it, at least for a casual hobbyist. But there are a couple of methods for determining gender though I would hold off on doing so until after the snake is comfortable and eating regularly and the added stress will not overwhelm it.

After you read the care sheets please post a description, and photos are great, of your enclosure if you have any questions. There is a great community here to help.