D&D =/= a real world simulation. Characters are capable of doing things that are impossible in the real world. We have these things in the game because they make the game, and the story within the game, better.
If a PC wanted to pick up a goblin and throw it at something, I'd look at the situation and ask, "Is the player adding something fun to the game, or is the player trying to cheat the system?" If they're not trying to do something overpowered, but instead are just having a good time and looking to do something cool, and it does not defy all reason, I'd allow them to try it and ask them to make an appropriate roll. However, I would not stall the action of the encounter too much to figure it out - I'd look for a simple answer.
How does that apply here?
The player tells me their enlarged goliath (who could clearly one shot a goblin with a single attack of their weapon) wants to pick up a goblin and just throw them as far as they can. I'd say, absolutely they can. I'd then ask what are they trying to achieve? Kill the goblin with flair? See how far they can throw it?
If they say they want to kill it in a fun way, I'd point out that they'll need to hit a barrier for the goblin to take the most damage, so throwing it into a nearby wall or obstacle makes sense. Then I'd have them roll normal weapon damage as if they attacked it and just reskin it. Why go through mental leaps for something that does not impact the game in the end? If they asked if they could throw it into a wall 30 feet away, I'd say sure without a second thought.
If they just want to see how far they can throw it, I'd spend 5 to 10 seconds looking up a benchmark on google. When I did that just now, I saw there was a 56 lb weight throw Olympic event in the early 1900s and the records were about 80 feet. I'd then tell them to roll a grapple to replace an attack as per normal grapple rules, and then roll 4 Athletic checks (plus 4d4 for the enlarge) and use that total for the distance they threw the goblin, and then give the goblin 1d6+strength damage for the rolling impact for their next attack. If I had not found something, I'd have looked up a hill giant boulder throw range and used that. I'd have obtained different results, but in the end - what does it matter?