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?