Does the extent of his power has been revealed?, and also how did he got his current rank by only getting goblin slaying quest?
Globin Slayer doesn't have any special powers unlike the Dwarves or Priestess. He only has his own strength, intelligence and his powerful individual skills. As you can see his ability at here, Goblin Slayer shown that he mastered a lot of skills such as:
- Trained Combatant (Goblin Slayer is relatively skilled in most weapons that he can get his hands on, using them effectively enough to kill goblins.)
- Master Swordsman ( Goblin Slayer is extremely skilled in wielding any bladed weaponry from short swords, daggers to knives.)
- Master Marksman (Goblin Slayer is an incredibly proficient archer; he can accurately shoot down goblins from vast distances.)
- Master Strategist (Goblin Slayer is an excellent strategist, specializing in ambush and guerrilla warfare. He comes up with extraordinary plans to trap large numbers of goblins, eliminating them with efficiency.)
- Goblin Knowledge (Goblin Slayer has devoted himself towards all matters related to goblins, ranging from their habits to their mindsets.)
- Lock Picking (Goblin Slayer possesses basic lock picking skills being able to unlock chests and to check for traps by using lock picking tools, though he mentions not to expect too much from his abilities as his class is not a thief.)
- Physical Conditioning (Goblin Slayer has rigorously trained his body to utmost perfection, granting him top notch physical abilities.)
- Strength (Goblin Slayer has a high level of physical strength.)
- Speed (He could strike down multiple goblins at once with superior speed and reflexes.)
- Endurance (He also possesses an incredible amount of endurance and pain tolerance, capable of fighting after having broken multiple bones.)
- Senses (Goblin Slayer is extremely aware of his surroundings, having extensively trained to utilize all his senses including sight and hearing.)
He also know how to use a lot of equipment as well.
He has slay multiple non-goblin creature also. For example like: Orge. In case you haven't read the novel, he has also slain other creature as well:
Dark Elf, Agent of Chaos, Rhea Scout, Evil Wizard.
Other than these, did you know that Goblin Slayer is a rank Silver in the guild?
Based on the ranking system, rank Silver is the highest achievement you can get in the guild.
For your question "How did he got his current rank by only getting goblin slaying quest?"
Slaying goblins is one of the lowest paid quests and a lot adventure doesn't like it because the paid but Goblin Slayer did not care. He only want to slay those goblin no matter the quest is getting paid or not just because the goblins in the story all are bad AND he doing this for 5 years. (Mostly by his own.) So you can imagine that he is very strong on the individual skill.
By getting his rank Silver, I think that the Guild Girl was the main factor that let Goblin Slayer get the rank. While mostly adventure doesn't care about Goblin Slayer, Guild Girl was the only one that keep helping Goblin Slayer. (I Shipping these two. :D)