Mouseover the bar on the left and use the mouse wheel. It will scroll but it's not really easy to see. 

Caravans give a food bonus (as a %), so unless you really want a road between two particular cities, always send a Caravan to a food-producing city.

The bonus is larger the farther away the source city is from the destination city.

The trade in the kingdom report is from treaties and such. Caravan bonuses don't show up there.

As for the bonus... it can be substantial. I've reached over 100% bonus on a single caravan which, on a city of a lot of base food production, is enough to sustain itself and then some. Note that the bonus applies to both ends so if you have two cities producing food and connect them, you would get the bonus applied to both cities. As well, a city can have more than one caravan send to it.

Give or take (I can check later), one large map square will probably give you in the 5-20 range. Two will be 40-60. At max zoom (at least for me) from one side of the window to the other, it's 100 to 120. 

I am almost certain that the bonus is based on road distance not physical distance so your mileage may vary.

Ask and I'll answer. I suppose at some point, I should write up a city guide to answer questions.

Can't

There are some spells that will change the tile.  But no Workers, etc.