How to get uniform sized biscotti?
Chill this or any cookie dough once finished mixing and before baking. Make your biscotti dough into perfect rectangle logs right to the edge of your pan, with even height and width right across. Use a ruler if you have to. I suspect you have a higher height in the middle - that dough will need to go somewhere when heated, and that's sideways (creating your longer middle baked width).

Once you have these perfectly formed rectangles of dough, chill in the fridge for an hour at least. This will really help keep it's shape and reduce any uneven spreading during bake. You could also try slightly increasing your first-bake temp 10-15 degrees as this will help quick set the exterior of the biscotti logs. The only other thing I may mention is the recipe - perhaps you can just slightly scale back the baking powder to help reduce the spread of the dough during baking.

Once baked just to set (depends on size), pull from the oven and let cool just slightly. While still warm to the touch, start slicing your cookies to a consistent, even desired width - I really recommend using an electric knife if you have one to make the work a lot easier for yourself. If you don't have one, just work quickly with your best serrated knife. Doing this while still warm will definitely help with reducing breakage and crumbs.