Assuming you are using the batteries that came with the radio, which are usually nicads, they should still have most of their charge 4 or 5 days later. A voltmeter with a load, such as this one, http://www.towerhobbies.com/products/towp1181.html , is helpful for confirming that the battery is ok. And if you're in doubt about the charge, put it on the charger for an hour or so to top it off.

Moving on to props, an 11x5 should give you better takeoff acceleration than a 10x6. However, it's only going to help a bit. The OS LA engines are good basic engines, but they are not power houses. If you want your Avistar to takeoff and climb significantly faster, you may want to eventually upgrade to a .45 size ball bearing engine. However, try the prop change first and see what you think.