It doesn't matter. You may have to fortify the beer's alcohol value if its only 5-6%. 7% is best 8% is better. I have always preferred dark rum for any fortifications but any hard liquor will do. The liquor that I use is 40% alcohol/vol. (Canada) Here is a calculation you can use;
Assume a 1 litre total volume (1000ml). Since we are now getting into calculations, I'm increasing the beer (substrate) volume to 200ml at 8% alcohol/vol. This means 800ml of vinegar base/culture + 200ml substrate (beer)
(200ml beer) (0.08) = 16 ml pure alcohol
16ml alcohol / 1000ml total volume = 1.6% alcohol to the total volume. This is a good value. Less than 2% but greater than 1%
Fortify 200ml beer that is 5% alcohol using 40% liquor to 8%:
(200ml)((0.08) / 0.4 - 0.08
= 16 / 0.32
Therefore add 50ml of liquior to the 200ml beer to get beer at 8% alcohol/vol.
Check: (50ml liuor)(0.40) = 20ml of pure alcohol
20ml alcohol / 250ml fortified beer = 0.08 (8%)
You will only be using 200ml of that fortified beer though. Save the rest.
As you can see, the finished vinegar volume will be 200ml. You must think of bottle size for storage. The bottle must be full (almost) and sealed tightly. You can put plastic wrap over the bottle top before sealing. Any hint of space and the culture will start to draw in air. This will maintain the growth of the culture which you don't want.
Re-arrange the values of the vessel volume and substrate addition to get the correct volume of finished vinegar for the storage bottle. Basically, he storage bottle volume will dictate what size fermenting vessel you need. Let me know if I've made some errors in calculations or you need further assistance. Always willing to help