There are a number of spiders that fit this description but it depends what country this spider is from.
UK spiders
If this spider if from the UK then it could either be a Mothercare spider or a Spitting spider. Both of these spiders are predominantly white in colour with black dots and markings across its body. The Mothercare spider is about 4mm long and can be found low growing vegetation. The Spitting spider is mainly found in southern England.
American Spider
If this spider comes from America,the only one that meets your description is the Carolina Wolf spider which can reach 35mm in length.
Australian Spider
There are a couple of spiders it could be that come from Australia. The Tent spider has a round white body with black spots running all the way down it.
The Opisthoncus best fits the description, although in some cases the body of the spider is more of a yellow colour. The Neosparassus species have many different kinds of spider all of which are white with black markings, some of which are more spot like than others.