I had to go through some trouble to figure this out because it's completely different from the original. Hope it helps people want to use custom portraits.
The dimensions needed for custom portraits appear to be:
Large: 210 x 330 px Medium: 176 x 271 px Small: 61 x 96 px
I tested this by creating a solid red portrait with a one pixel green border and changing the size until it fit exactly - enlarging it by a pixel one way or the other would make the green border disappear. The game appears to do some sort of anti-aliasing around the edges of the portraits, so the green I'm seeing may be bleeding over from that. Perhaps rounding the numbers to the closest 5 might give you better results:
Large: 210 x 330 px Medium: 175 x 270 px Small: 60 x 95 px
The portraits should be in .bmp format and should be placed in My Documents/Baldur's Gate - Enhanced Edition/portraits. Naming the files appears to be the same as it used to be - 7 characters max, ending in L for large, M for medium, and in S for small. As such:
About the small portrait: The number I listed above allows it to appear normally in the character creation screen. However, it doesn't work so well in the party list during gameplay. I had to shrink it to 61 x 89 for the border around the portrait to appear properly.