This can not be answered with a single number. In RuneScape, the amount of experience from combat depends entirely on the amount of damaged caused. This, in turn, depends a lot on your combat level, possible boosts (especially an attack or strength potion), and the items you are wearing, especially your weapon, but also - to a lesser degree - armour, amulets, boots, etc. - in almost any item slot you can use items that boost your combat.
It doesn't matter so much whether you attack giant Spiders or something else. I do suggest you attack something that has at least 1/2 your combat level - that way, the creatures will be aggresive (as soon as you kill one, the next one will attack you), which increases the xp per hour. Also, attacking very low-level creatures wastes time, since you can't cause a creature more damage than its total health points.
On the other hand, you wouldn't want to attack creatures that have too high a level, since in that case, you'll need more food. This will not only cost you money, you'll also spend time going to the bank for more food.
Also note that you can get defence experience from all three types of attack (magic, melee, ranged).
If you have a paid membership, you may want to do Slayer tasks - simply to get experience in one more skill. That is, while you train combat, you also train Slayer.
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