Tir Avalon House Rules

Target modifiers are based upon the size of the target, and the part you want to hit. hitting a leg is easier than hitting an eye for example. So with that in mind, here is the basic premise.

To hit a limb on a creature, is a -4 attack roll if the creature is the same size as the player. This is reduced by one for every two size categories the creature increases in size, and the opposite if it is smaller.
To attack a hand, head, foot for example is a further -2 to the attack roll, with the same modifiers.
To go for an eye, is a a -8 penalty, again modified by size.
The speed of the target effect the attack roll when the target is making a dash action (not disengage etc), o if they are just flat out running (double movement) they are slightly harder to hit, so they attract a -1 penalty to attack as well.
Targets at long range, also gain a -2 penalty on a called shot.

Examples:

Elf shooting an arrow at a Frost Giant’s eye. The start of the attack gains a -8 to hit, however the size reduces that down to -7.
Human Ranger shooting at an imp to wing it (literally). Wings count as limbs, so a tiny creature’s wing would calculate as -4 to hit, as well as another -1 for the size, making it a total -5 modifier to attack.
Dwarf crossbowman shoots his bolt to hit the knee of a running Hill Giant in the distance to drop him, modifiers calculate as: -4 for called shot, Huge creature reduces the penalty by 1 to -3. Going for the knee makes it -5 to attack, the target is also moving fast, so the attack is now at -6, and it’s also long range, giving a final -8 penalty to hit a running Hill Giant in the distance, and unless they have Sharpshooter or some class action to remove the penalty, the long range shot is at disadvantage as well.