My answer would be if you are welding two pieces together, the filler rod should be somewhere in the neighborhood of 1/3 of your welding puddle size, or maybe even smaller like 1/5 of your puddle size if you want the frozen weld bead to be wetted out looking and not raised up very tall.
If you are doing tall build-up (e.g., building up an edge), the filler rod should be closer to 1/2-1x your puddle size, to help freeze the puddle faster.
For a given puddle size width, a larger diameter filler rod will freeze the puddle faster, and build the profile of the weld bead up. If your rod is way too big for the puddle, you will have a hard time even getting it dipped into the puddle - (i.e., you'll try to dip but miss the puddle.)
If your rod is too small, you'll have to feed too fast. The puddle width can then grow out of control. (Plus with smaller rods you need to feed fast, you'll run out of rod very quickly, have to reach for new filler rods more often.)
'13 Everlast 255EXT
'07 Everlast Super200P