The Video Recordings Act (Labelling) Regulations 1985 state the letter or number (U, PG, 12, 15, 18, R18) within the triangle, circle or square must be no less than 5mm in size. The size refers to the letter or number and does not include any shadow or other design elements.
For VHS packaging the symbol must be shown on one of the largest faces (normally front cover), the spine and on another of the largest faces or another face other than the spine which includes the explanatory statement within a rectangle (normally back cover).
For DVD packaging the symbol must be shown on one of the largest faces (normally front cover), the spine (except if it is less than 2cm) and on another of the largest faces or another face other than the spine which includes the explanatory statement within a rectangle (normally back cover).
In 1998 an amendment to the Labelling Regulations removed the requirement of a coloured background (U-green/PG-yellow/18-red/R18-blue) to the on body symbol of a single sided disc. This means the on body symbol can be displayed in black or white. The minimum 5mm size applies to the on body symbol.
A further amendment to the Regulations addressed the issue of double-sided DVDs/VCDs which allowed for the symbol to be replaced with 'UK - U/PG/12/15/18/R18' on one face of the disc and to be a minimum height of 2mm and be black and white. This height amendment applies to the double sided disc alone and not the packaging in which it is contained which must still comply with the 1985 Regulations.
The unique registration number (comprising the BBFC classified title, register number and category symbol) which is issued by the BBFC to the UK rights holder must also appear on face label of the VHS/DVD, unless the DVD is double-sided, in which case the above paragraph applies.
A full transcript of these Regulations can be obtained from Her Majesty's Stationery Office or your local library.