Why isn't there an "ideal value" for a given currency? If a currency can be worth too little (e.g. needing \$10,000,000,000 to buy a loaf of bread) and worth too much (e.g. being able to buy a loaf of bread for $0.00001), why isn't there an "ideal value" (a point, range, or a shifting set of priorities based on market conditions) that a currency could be fixed or drawn toward?  

Many everyday transactions are still done in cash, requiring mental calculations from the transacting parties. Too large numbers and too small numbers create transaction costs that, at least for the extremes, cannot be ignored.  So conceivably, some "middle" point could be considered "ideal" as regards the efficiency of money as medium of transactions.

What forces may prevent such an "ideal value" for a currency from being established?
Because money is really just arbitrary numbers, and those numbers only really have any meaning in relation to the other numbers in the set. For example:

Average salary is \$50,000 and a entry-level car costs \$10,000. If the price drops by \$5,000, it's well known that sales will go up, ceteris paribus. But what we increased all prices and all wages by a factor of 100,000? Would that change people's behavior in an otherwise identical system? E.G. now the average salary is \$5,000,000,000 and the average car costs \$1,000,000,000 and the price drops 500,000,000, then what happens? 

Well there is a fancy economic name for the theory that covers this (which I forget at the moment), but the answer is No, nothing would change. It's quite obvious that humans are very insensitive to absolute price and very, very sensitive to relative price. (There have been several economy-wide currency revaluations in other countries that serve as examples of this.) Adding or taking away a couple zeros at the end of the numbers doesn't change the really make a difference (provided you apply the change uniformly to all numbers everywhere).

The two major problems that hyperinflation a la Weimar Republic cause are 1.) governments are slow moving beasts and don't react quickly enough to start printing 1 billion dollar notes, etc. so you end up having to use a wheelbarrow full of cash for purchases (which in today's economy isn't as big of a concern since almost all money is just electrons in a computer somewhere,) and 2.) changing prices and adjusting to changing prices and adjusting your forward looking predictions about prices are all not costless actions. It takes time and energy to do any one of those three things. Because of that fact, it imposes a very real dead weight loss on the economy as a whole when prices increase or decrease very rapidly. The faster things move, the more costly it is to adjust and the more often you have to do so, so the DWL becomes exponentially greater than a more slowly-paced situation.