The aim of this study was to explore the relationship between pharmacy size and the likelihood of obtaining antibiotics without medical prescription at a pharmacy. In 2008 in Catalonia, two actors presented three different cases in a randomised sample of pharmacies and asked pharmacists for an antibiotic. Pharmacies were considered as small when having limited space with only one counter and a maximum of two professionals selling medicines, as medium sized with three or four attending professionals, and as large with a large selling space and more than four attending professionals. Of the 197 pharmacies visited, 88 (44.7%) were considered as small while only 25 (12.7%) were large. Antibiotics were obtained without a medical prescription in 89 (45.2%) pharmacies, mainly in small pharmacies (63.6%), followed by medium-sized pharmacies (35.7%) and large pharmacies (12%) (p<0.001). Large pharmacies, that probably have a greater income, more closely followed the prevailing legislation of not selling antibiotics to patients without a medical prescription. This observation should now be confirmed in other countries where over-the-counter sales of antibiotics are prevalent and should be taken into account by programmes aiming at achieving a more prudent use of antibiotics.