Take my advice unless your luggage will fit into something the size of a crisp packet (and I don't mean a multipack), book your luggage into the hold when booking, or even better use a different airline with an acceptable baggage policy.
I'd only ever used flybe once prior to my current trip and on that occasion I used a much larger case than my current "cabin bag" size case so I booked it into the hold at extra cost in advance. No problem. On that trip others in my party were measuring their cabin luggage in the "sizer", so as a matter of interest I measured mine too, amazingly it fit so on that occasion I'd paid the extra hold luggage fees for no reason.
Fast forward 4 years and I'm travelling with a much smaller case which is definitely "cabin bag" size and 5 minutes before the plane is due to depart I am held at the gate to pay a £50 excess baggage fee along with at least 3 other people (the gate was only announced 30 minutes before take off as usual and no movement was made to board passengers until approximately 10 minutes before takeoff!). The ground personnel did make the announcement that they were employing strict rules regarding the baggage, in fact that was their main announcement (which tells you everything you need to know....."we want cash, cash, cash"). Unfortunately these "strict rules" were inconsistent.....
During my time at the gate I witnessed a passenger with a standard wheelie laptop case who was unable to fit said case into the "checker" simply receive a "passed for boarding" sticker (the case was too deep and wide but definitely within the height restrictions as it was only about 20cm high, if that). Various passengers seemed to board without being challenged including a passenger with a holdall the size of a 7 year old child. A passenger whose case would not fit but also received a "passed for boarding" sticker as when they tried to charge the passenger for excess baggage the computer kept saying there was nothing to pay. The staff then seemed to converse with someone via telephone and "hey presto" the next in line for excess baggage was offered a charge of £100! They hastily reduced it to £50 which seemed to be the standard cost thereafter for those of us who were deemed to be have excessive baggage.
When we challenged the gate personnel on the inconsistency and the size of the "checker" they became very defensive (understandable) and quite rude - obviously having to implement this kind of rule is having an acute effect on front line personnel.
I found the whole episode very distressing and again on our return journey I was stopped at the gate to pay for excess baggage as the system had obviously logged the outgoing payment charge and expected us to pay extra again on the return. However having been stung once we sent our bags home ahead of us by road - once bitten, twice shy. You may have had an extra £50 from me flybe but you're not getting £100!!!
I would be interested to hear if anyone has actually measured the size of the "checker" as I have to say if a small laptop business case won't fit then there is little hope for anyone else.
I will definitely not be flying anywhere with flybe again, not a pleasant experience at all when you're treated like a criminal (and the personnel act like prison officers to boot).