For non-vintage house style champagne, stores with high turnover are good because you know the inventory has not been sitting on the shelf for extended periods. Some Champagne Houses have started to put disgorgement dates on the bottle. The dates are either actual dates or dates available through a QR code (scan the 2D barcode with a mobile phone for the pertinent information). The date will give you an idea of the actual age of the wine and how long it has been on the market.
For top quality champagne, try to find a store with a temperature controlled storage unit with low lighting. If there are bottles on display in an area with bright lights, find out how they store additional bottles and if storage conditions are acceptable, ask for one from there.
Avoid having wine shipped in hot months. This applies to any wine but especially to champagne and other sparkling wines, which are somewhat more sensitive to temperature fluctuation and heat than others. It is also a good idea to minimize purchases at the end of a hot summer unless you can verify proper storage, when shipments were received and how they were shipped.
When considering the size of the bottle to purchase, the best formats are generally the standard 750 mL bottle or the magnum (1.5 Liters). Standard bottles are the safest bet, but the magnum is actually an ideal size. Refer to the webpage Champagne Bottles for additional information about bottle sizes.