From a very basic view, the length that your extending your wires would have no great impact on anything of the sort if your just extending the length to clear the wheel arc.
The fuse box will be before any great length of wire that could potentially cause a fire which is the main purpose of the fuse box.
Also you should be matching the same gauge of wire that you were extending. Make sure to do it correctly and not jank ass with electric tape and twisting the wires. This is the area of your car you should take extra care to make sure is done correctly. Use either insulated crimps or solder and heat shrink tubing.
Here is a resource that might be more then you want but has pretty much everything related to creating or modifying your wiring harness.
