Funny you mention eternal hell at the end.
Let's follow two logical paths based on that and lead on toward the 'value' of Christ's atonement.
Assume the wicked spend eternity in hell, suffering for their unrepentant ways.
1) The sins of one person are worth an eternity of torment whether there be one sin or many
2) And surely more then one will suffer in hell and more then one will be redeemed for heaven
3) And an eternity of suffering is required for each because each is a sinner whether redeemed or not
4) Therefore Christ's atonement must be worth eternity of suffering times the number of sinners, which is a number that is not real and cannot compute.
Assuming hell is a place where the wicked suffer eternally for their sins cannot logically compute; infinity times anything is the same number.
Now assume the wicked do not suffer torment eternally, but are most definitely punished
1) The sins of one person are worth something and will cost that person something whether there is one sin to pay for or many does not matter
2) Christ covered that cost because he suffered for it already if you choose to accept it or not
3) Therefore, one sacrificed Lord is worth the cost of suffering for the sins of the world.
Now in my opinion it is semantic after that to say that Christ's atonement is limited or unlimited. But here is how that works for both scenarios.
Eternal Hell as punishment can give Christ limited or unlimited atonement depending on the view of freewill, predestination, etc. For the predestinationist the saved were meant to be saved and the damned were meant to be damned, hence, Christ's atonement only covers the saved and is therefore limited. For the free-will-ist, all were meant to be saved, but not all will be, hence, Christ's atonement covers all and is therefore unlimited, and chalk it up to antimony how the wicked still suffer for eternity for eighty years of sin.
Less then Eternal Hell as punishment naturally makes Christ's atonement limited, but only because of its limited use. As an analogy if you have $100 and buy something for $50 then the value of your expenditure is limited at $50, yet you could have spent $100. But that is hardly important; the point is that you spent $50. Likewise, Christ's atonement bought salvation for all.
As a side, I am pretty sure that ancient Jewish sacrifices had a value. If I recall one lamb or ram could cover the sins of ten people for a year. A dove and other lesser creatures did not necessarily covered sin, with exception of the poor, but showed God your desire to sacrifice to Him and for Him. However, there are clear examples in the OT where the sacrifice for the absolution of sins is not needed for the credit of righteousness. Most notably, "Abraham believed God and it was credited to him as righteousness." Compare Romans 4 and Genesis 15.
So you might now question what less then eternal hell is. I personally believe that "the wages of sin is death" as the Scripture says. John 3:16 says that to keep us from perishing Christ died for us. I can expand on this more if you want but that is beyond the scope for the answer to your question.
Disclaimer and expansion on logic of the question
Yes, my answer is largely opinion, however, I think it is well known that Lutheranism preaches eternal hell and it is hell that atonement saves from, so the discussion in my post explores the logic of both sides. The Lutheran must claim unlimited atonement because they also claim eternal hell and all might be saved. So, yes, I did sort of fail to answer the question, but that is because you cannot answer "How is something unlimited?" Instead you can only try to understand it from our limited view. Further, unlimited atonement basically means the same thing to most Christian theologies; it is not an exclusively Lutheran doctrine, which largely comes from Roman Catholicism, the oldest organized denomination.