Fossils can be simply be considered as marks of the organisms that lived many years ago but donot exist now, but their marks exist like feces or footprints. They provide us evidence of organism's survival as well as evolutionary history and relatedness with other organisms. For example: Fossil from a specific layer of earth can tell the geologic timescales. They tell a story that if one organism was found in a certain age then which related organisms emerged soon after that age. It is a useful way of studying evolution.
2) DNA evidence:
All the DNA of every organism is composed of unique sequence of nucleotides which have a lot of importance in the evolutionary studies. DNA sequences of organisms mutate over the course of time which changes the organism's genetic study and evolves them into new specie. Therefore, this sequence of DNA can help in finding the ancestors of an organism and also that which organism is most related to a specific organism. This approach of studying evolution is called Phylogenetics.
3) Case studies:
Different evolutionary case studies performed by scientists can also provide evidences of evolution. For example: The Darwin's case study on finches of Galapagos Island helped in assessing that how evolution occurred when finches were isolated by geographical barriers.
Though some cells are bigger than others, they do not scale with size. For example if you look at skin cells on a dog they would be similar in size to a skin cell from a human. When an animal or species is bigger it is usually just made from more cells.
