Hey!
Airlines generally specify the required minimum height on the requirements section of the job info section (for eg: are you at least 18 years of age, can you swim 20 meters unaided etc...) If the airline has already offered you a job then I would find it odd if they were to then measure you during training!
If I were you, I would consult Jet2 as to whether they give a little leeway on the height issue before you give up your job
These are just my opinions on the matter!
Hope that helps!