Looking at the emails we have had from Avro, they state a ridiculously small size of hand baggage in terms of dimensions.
Unfortunately you do not state how ridiculously small the dimensions are.
From the Monarch website, all flights with MON flight codes have a 5Kg hand baggage allowance with dimensions not to exceed 56 x 45 x 25cm which is a reasonable size for the weight allowance. Also remember, your 5Kg hand baggage allowance
is part of your 20Kg hold baggage allowance.
Enjoy your holiday.