Three rovers side by side, the humans are added for scale. In Robots size does matter as bigger means easier management of the terrain and that means travelling greater distances in the mission time.
From left to right the following rovers are seen on the above image:
Sojourner which drove off the Pathfinder lander in 1997 was about as small as a VCR (do you still remember those?) on 6 wheels. It travelled all in all no more than 60 meter from Pathfinder. Staying close, examining every rock around the lander, naming every rock too.
Spirit and Opportunity travelled many kilometres, but started small with short drives from their landing platforms. Being slated to only operate 90 days, perhaps 180 days, they were not expected to travel kilometres. At first also naming every rock they examined, but soon driving farther and farther from their landing site. They were about the size of a small refrigerator turned on its side, again 6 wheels pushing their way through the often loose Martian soil.
Spirit operated for 7 years, limping for a long time as one front wheel gave out in the early years. Still it did reach Husband Hill even if it took a full year to traverse the Martian plane of the Gusev Crater to get to the Columbia Hills, of which Husband Hill is one.
Opportunity is still operational 8.5 years after arriving on Mars. Travelling from the small (20 meter diameter) Eagle Crater where it bounced and rolled into after landing, to Victoria Crater, where it drove around for a year before the engineers chanced sending her down into it, expecting her to never get out again. Opportunity surprised us as she did manage to climb out again. Basically: hooray for the superb human engineers and software engineers writing her code to accomplish that feat.
The Mars Science Laboratory named Curiosity, is larger still. As large as a car. Thinking family sized car as 3 by 3 meters wide and 2 meters high is large if not long for a car.
How large she is can be seen clearly on the above image where two humans are standing among the 3 type rovers that we send to Mars over the last 15 years. See below for another comparison of the MSL with the height of an average humen.