you can use a higher number of bars, however, by changing the periods you can use a smaller number of bars for testing.

I think you may find that 50,000 bars is going to be a bit slow to obtain results...... unless you have a machine that is far faster than any that I know of.

The more bars that you are testing will cause the ea to produce less per bar, ie if you are using 5,000 bars you will get a profit of 'x' and if you test 50,000 bars your profit will be substantially less than 'x', (referring to the generator).

You may wish to consider using 30,000 bars and then testing against several periods to determine the effectiveness of your logic.

And when trading. reoptimize often so as to reflect the current market conditions.

Others may have a different opinion and I would be pleased to see what they may have to add to this.

If you are still sure you really want to increase:
Market > Data Horizon
then it has a field for Maximum number of bars. I agree withe Blaiserboy, mostly I use around 25,000 bars for testing. I only use more than that for a long, long term perspective and don't want to manually swap between 2 data files for OOS testing.