I usually wear a little more snug during the day to keep my diaper from slipping down and requiring "adjustments" periodically throughout the day. At night however, I add maybe an inch around the waist to get a more comfortable fit for overnight.
I'f you're having problems with "diaper slippage" during the day, I'd suggest you wear something with both front and rear waistband elastic. Velcro also makes adjustments much easier, and you don't have to worry about the tapes popping after you've moved them a time or two. (iirc all the velcro diapers available right now have front and rear elastic) It's very annoying to be diapered at work and have a tape that won't stay taped because you've moved it once or twice and now it won't stick properly anymore. Tykables has two different products out right now with velcro, the Waddler Overnights and the Little Builders. I'm trying out the Little Builders now, considering them for the position as my new daytime diaper. They're a thinner version of the overnight, making them a really good daytime / at-work diaper. (check out the reviews on their website, pretty much everyone is saying they're "the daytime diaper we've all been waiting for") Those are your thin and medium thickness options. Bambino's Magnifico is the thick velcro option. All three of these are smooth plastic shell btw, and I have yet to see a tape rip from the back of any of these models. (something that does happen from time to time with most adhesive tapes I've seen when taped tight)