Hi

. . . . can you set a length for all transitions from the settings . . . .

That depends on what you mean:

If you want to globally set all transitions to the same length for the program - then no you cannot do this.

Some transitions are longer than others and if you were able to set them globally to a preset length then you would likley find yourself constantly changing some of them in a project.

If you want to set all the transitions between objects on the timeline, in a project, to the same length then it is possible within limitations - ie all the transitions between object must be of one type. 

To do this set up the transition between the two objects then click the transition button (arrowed) on the second object and select one of the options circled as shown below

I use this method with one main transition - usually crossfade -  then change individual transitions as required to split up the story I am telling - eg fade through black for a change of place or time - I do not like watching videos that use transitions for transitions sake.

HTH

John EB