How to modify dimensions?

Is there a way to modify a figure’s dimensions by typing them in somewhere because I notice measurements are multiples of 5. For example I want to create a line that measures 50,5mm but the software make a line with 55mm of dimension.

Yes, there’s a way but it would be a whole lot easier to give you an exact answer if you could share a sample LO file that demonstrates this. It could be a simple as turning off grid snapping in the Arrange menu.