This is an extensively tested X-ray film bag with effective lead content of up to 0.5mm, highest amongst standards for radiation shielding. 

Small: A flat 21x20cm bag (500g) with 0.35mm effective lead content, good to fit about 10-15 rolls of 135 or 120 film for day use.

Medium: A 22x18x7cm portable bag (825g) for short trips with 0.5mm effective lead content, good to fit about 30-40 rolls of 135 or 120film, alternatively, a standard SLR camera body/two film backs and 20 rolls of 135/120 film. 

Large: A 30x21x9cm bag (1.35Kg) with 0.5 mm effective lead content, a choice for long term travelers fits about 70-80 rolls of 135 or 120 film, alternatively, a reasonably sized medium formate camera with 20-30 rolls of film.