How can I define the size or aspect ratio of the result? In the end, I need a graphic 850px X 310px.
That is not possible in the general case. Only layout algorithms Tree -> Compact and Orthogonal -> Compact support aspect ratio hints (i.e. the algorithms may violate the specified aspect ratio).
How can I reserve a particular area in which nothing may be located?
Create a dummy node the size of the reserved area, run a layout algorithm, use "Tools" -> "Geometric Transformations" to move the graph in such a way that the dummy node lies at the desired location (if necessary), and finally remove the dummy node once again.
How can I customize the format for general arrows. Or I have to do for each individual?
I am not quite sure what you mean by that.
Do you want to customize the edge that is created when dragging the mouse from a node that is not selected to another node? If so, customize one edge, then create a user defined palette section (using "Edit" -> "Manage Palette"), right-click the custom edge to open its context menu and choose "Add to Palette", go to the palette section and double-click the corresponding edge template (should result in a blue background).
Or do you want to change existing edges? If so, select the edges you want to change (first select one edge by left-clicking, then press and hold SHIFT while left-clicking additional edges) and use the properties grid in yEd's lower right corner to custmize the selected edges.
