Laminate flooring in multi-width bathroom
I second @ChrisCudmore's recommendation of not using laminate in a wet environment. Consider choosing a flooring type that is less susceptible to degradation via moisture.

That being said, the rule of thumb for determining plank width is to shoot for your starting and ending widths to be greater than 1/2 a whole plank width.

Here's the calculation process:


Measure the width of the span
Subtract twice the recommended expansion gap
Divide by the width of the plank
Take your fraction of a plank remainder, add 1, and then divide by two.
Multiply by the width of the plank, and there's your starting and ending row widths. 


For multiple width rooms, do this for all the individual widths, and then pick the option that leaves you with >1/2 a plank width all around.

So for example, if you have a room with different widths of say, 67-1/2", 83-3/4", and 71-3/8", using a 5" wide plank you'll end up with starting plank width recommendations of 3-3/8", 3-5/8", and 4-1/16". For example, using 67.5" with 5" wide planks and a 3/8" expansion gap: 

(REM((67.5 - 0.375*2)/5)+1)/2*5 = 3.375


Obviously you can't use all of those, so pick the smallest that doesn't send your largest width over 5".


If you pick 3.375" starting width, your other ending widths will be 3.375", 3.875" and 4.75". This option works, since none are below 2.5", and none are above 5".
If you pick 3.625" starting width, your ending widths will be 3.125", 3.625", and 4.5". This option also works.
If you pick 4.0625" starting width, your ending widths will be 2.6875", 3.1875", and 4.0625". This option works, but it's pushing a bit closer to the &lt;2.5" width range, so I'd skip it.


So for the above scenario, I'd go with a 3.375" starting width, since it maximizes your minimum starting/ending plank width for the entire floor.