Apart from making sure that your rose is kept adequately moist and being sure to remove the spent blooms as soon as they start to fade - there are several other reasons why roses sometimes produce buds that are either fewer in number or much smaller than they should be - such as when a rose has recently been transplanted and is in the process of settling in and putting its roots down.
Extended periods of extremely hot weather can also cause roses to have smaller than average buds - as not all varieties cope with extreme temperatures and therefore conserve energy by putting out smaller blooms or it could be - that as not all rose varieties flower throughout the year - you may have one of those varieties that only flowers at a specific time of year - which as they come to the end of their flowering time tend to put out much smaller blooms.
If its none of those - then the only other reason why a rose may put our smaller blooms is due to lack of a potassium rich fertilizer - which is generally best applied at the first signs of growth in the spring and then periodically throughout the growing season