Measuring stem clamp / handlebar diameter
Measure the handlebar itself, while unmounted from the bike, at the center most point of the bar. Use a digital or vernier scale caliper. 

The bar from the 2009 Bianchi Cortina is a 25.4mm MTB handlebar, with about 2 inches of rise. I expect yours will be as well. Do not use a 26mm bar, although you can likely mount it on the stem, because the installation will damage it, and it will fail, probably at some future point when you are riding it. 

If you want a drop bar, you will need a new stem as well as handlebar. 25.4mm road drop bars are rare or non existent, since 25.4mm was the mountain bike standard. 

I hope that helps.