The -azo/za suffix has four different functions:
- As an augmentative: perrazo, manaza (a big dog, a big hand).
- As a pejorative: aceitazo (dense and turbid oil).
- It can express a hit given with the object represented by the word to which it is appended: martillazo (a hit given with a hammer), almohadazo (a hit given with a pillow).
- Sometimes, it can also express a hit given the object represented by the word to which it is appended: espaldarazo (a hit given in the back); it's worth mentioning that espaldarzo not only stands for the hit given in the back, but also for a sign of recognition and aprovement (see example below).
Here are some example sentences:
Rompió el bastón con sus manazas morenas (She/he broke the stick with his big brown hands).
Mi esposa no quiere ese aceitazo ni regalado (My wife wouldn't accept that turbid oil even if it was free).
Todos recordamos el zapatazo propinado al presidente iraní Mahmud Ahmadineyad (we all remember the hit given with a shoe to Iran's President Mahmud Ahmadineyad).
Monti recibe espaldarazo del Parlamento italiano (Monti receives support from the Italian Parliament).
In your concrete examples, zapatazo y golpazo, the particle -azo acts as augmentative.
Some of the rules governing the addition of the -azo/za suffix are:
Words ending in a non-tonic vowel, loose the vowel:
Words ending in a consonant, simply add the suffix: