Are there any minimum sizes for fire doors to remain compliant? My kitchen doorway  is narrow (670mm) and short (1964mm) Also what overhead door closer would be best for a door of this size?

PS.  I have found a solid core FD30 blank door that can be trimmed excessively providing it is re-lipped.

You will find about minimum widths in approved document M page 7 check out https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/540330/BR_PDF_AD_M1_2015_with_2016_amendments_V3.pdf. As it is an existing door the BCO/AI may consider a smaller width but that will be up to him/her.