During the G1 phase the cell grows in size while also synthesizing mRNA and proteins. This is all in preparation for the following steps that lead to mitosis. The duration of the G1 phase is different in various organisms.
In humans, the G1 phase lasts about 6 hours, while others have such a short phase that its barely existent. Various factors can influence the phase, such as nutrient supply, room for growth and temperature. A good example of this is a plant only growing as large as the container it is planted in due to lack of room to grow. Fish kept in tanks also grow to the size of their environment.