in addition to the above answers... I think, besides the indication of all kinds of sizes of pandas... by using doubled characters, it implicits there are MANY of pandas of all kinds of sizes...

in general,it means"all kinds of "or"diverse" instead just "big and small ones". why are the characters doubled? cuz it's a fixed expression,there are many other AABB words or phrases,too.such as 干干净净,漂漂亮亮,兢兢业业(hard-working)明明白白,家家户户. i think it is to emphasize and exaggerate the emotion and affection when they are doubled. hope it will help:)

If they are not doubled, then it's 大小, and 大小 means "size". e.g. 那只熊猫的大小 that panda's size You can interpret 大大小小 as big ones and smalls ones, but its focus is actually on that there are pandas of different sizes. e.g. 里面有大大小小的熊猫。There are pandas of all kinds of sizes in there. Similarly you can say 高高矮矮的建筑 (buildings of different heights), 胖胖瘦瘦的人 (people of different weights). Same as 大小(size), 高矮 means height (but not altitude). According to the rule, 胖瘦 means one's size or weight too, but in order to avoid making sensitive people uncomfortable you can just use "weight" because you don't know they want to be fatter or thinner.