Just curious as to how a Airline decide what aircraft to fly on a particular route.For instance how do they decide to Fly a smaller boeing 757 vs a Bigger more comfy 767 or 777 aircraft on long flights to europe?

I’ll be going to Europe in a few weeks and I’m not yet in that excited Europey-vacationy mood. I know there’s sooo much to see and do in Europe. But, I just don’t know how to approach a vacation to make the most of it. Should I wake up each morning in Europe and have a goal to do?

I read some in a travel book and the warnings were so scary concerning assaults, weapons and kidnapping. I am now a little shy of travelling there and yet would like to go if it is safer than what I read. Anyone help?