You allocate whatever budget you are happy spending. Google will then give you as many impressions and clicks as that budget allows at your bid level.
My recommendation would be to start relatively cautiously - allocate less than you anticipate spending - since at the outset you are likely to discover that an amount of your budget "leaks" on bad traffic. Fix this before you open up your full budget.
Decide what you want to psend a month, divide it by 30 and that will be your daily budget. Google may spend up to 1.2x your daily budget on any day, but will balance that by the end of the month so that you stay within your overall monthly commitment.