A portion of nuts is classed as what can be held in the palm of your loosely cupped hand, which amounts to roughly an ounce as per the packaging advice.
Naturally I ignore this and scoff away to my hearts delight!
Take no notice of product serving sizes. Sellers have an incentive to make you think your unit of product will do for much more than it actually does. For example, a can of soup or a frozen meal are both single portions not two. And we don't need to be fooled by nutrition information for only half the product.
OG that depends. When its something like cake or other junky type food, the seller has got a vested interest in presenting it as “healthier” than it is. So for instance the pack will show a slice of cake that is a sixth of the whole cake but if you read the nutritional info on the back of the pack, especially the calorie and sugar info you will find that they state that a portion is an eighth or even a tenth of the cake.