This might seem obvious, but its surprisingly easy to get wrong. Before you set out to buy a bag, write your entire packing list out first. This will help you gauge how much you need to pack as well as what bulky items (if any) you need to consider.
For example, if you already have a small backpack that you frequently use on weekends away you might think that this will suffice for an overnight trip away for a wedding. It’s just one night so you won’t need to fit much in. What you may not think about is keeping your shirt and suit pressed while it’s stuffed in with your dress shoes, toiletries, laptop and sleepwear. What would be better in this situation, is a small hard-shell suitcase that neatly fits a garment bag with minimal folding.