you can set the max texture size in the render settings to help with the memory error.

How big is too big depends on the type of a rendering you're making.  Think about how many image pixels will your character cover.

A face closeup on a large image, especially if it's for a print and you want fine detail, 4000x4000 is not too much.

If the whole character is covering about 300x200 pixels on the image, even 1000x1000 may be too big.

Hi, my namez: "NO, Bad Kitteh, NO!"  Whaz yurs?
BadKittehCo Store  BadKittehCo Freebies and product support