Until relatively recently, almost all of my plants were grown on windowsills
Big windows = Room to Grow!
Big windows are great for mature Nepenthes - some species can get quite large! Young plants form flow growing rosettes of leaves and lower pitchers for several years before they start climbing. Overall size will depend greatly on the species. You may need to get creative with bamboo stakes, curtain rods, and hanging pots to give you Nepenthes room to grow.
Left: Growing on windowsills does not mean smaller pitchers! Nepenthes x Dyeriana loves being in the living room!
Most important - Light!
It's amazing how much variation there is in light intensity on the same windowsill. Just moving plants a foot closer or further to the window can be the difference between robust growth or sunburn. Pay attention to your plants - a slight red blush to the leaves and compact growth/appearance in the lower growing, rosette stage are ideal. I experiment moving my plants around the windowsill to see where individual plants seem to grow best - however keep in mind it takes about a month to really see changes in growth due to light. Expect pitcher production to slow or even stop in winter with reduced light, but resume with vigor in the spring!
Right: Nepenthes maxima constantly puts out great pitchers on the windowsill!
Humidity - probably the toughest factor when growing plants in open air in the home!
All my humidity loving plants sit on seedling/propagation trays filled with about an inch of black lava rock. These trays catch overflow from watering and provide a lot of surface area for the water to slowly evaporate, increasing local humidity. I also mist my plants once in the morning and once in the evening. The evening misting is particularly important of boosting humidity all night long. If your plant is receiving enough light but is not producing pitchers, low humidity is probably why! Investing in a small personal humidifier might be worth it if you live in a very dry climate.
Left: Notice the large trays of lava rock that help boost local humidity for my smaller plants in the rack