Add the HP Sprocket 3x4 Instant Photo Printer to the list of highly portable printers that can serve nicely as a cell phone or tablet companion. Like most of its competition—including the Kodak Step Instant Mobile Photo Printer, our current top pick for wallet size (2-by-3-inch) photos—the Sprocket 3x4 is small and light, includes a built-in battery, connects without wires (using Bluetooth, in this case), and is reasonably inexpensive. (It's $129.99 on Amazon at this writing; list price is $159.99.) Where it differs is that its photos are 3.5 by 4.25 inches. If wallet-size is too small for your tastes, while the printers for 4-by-6-inch photos—like the Canon Selphy CP1500—are too big and heavy, that puts the Sprocket 3x4 in your portable-printer Goldilocks zone.