At any point of our lives, we have learnt to hone our sense of fashion through the many walking examples surrounding us. The general idea of how we come to decide and select on a clothing item is through the lens of others. Don't be like that, just go with your own ideas and be comfortable. Nothing too tight, let your body breath, think about temperature more than shocking fashion fads.
If you were to walk down any street, you would see countless clothes pairings by different individuals. Although it may be an unconscious action, we store the outfits we have seen and keep it as referrals for another day. This may also occur when one reads fashion magazines which advices its readers of what designs can work in different scenarios. When facing a choice between medium-sized prints or large-sized prints, your mind works its way to dig into stored images or advices that would help you decide on the fashionably right selection. If that fails, one simply brings both outfits into the dressing room. That would equate to more time trying out items you may not buy (if the cut is simply unflattering) if you are on a time constraint.
In the event that the changing room mirror fails to tell you of the better choice, don't forget that you can always ask for a second opinion. It is always a good idea to have a companion when you are shopping for a new outfit. An outsider's view may be able to point out certain angles that may be unflattering or even financially help you decide when there is only enough for two outfits instead of there. There are the changing room assistants who can offer their opinion but always take recommendations with a grain of salt. Ultimately, it is you who will be wearing the outfit.