I'm working on PEG 6000 and the size of the plasmid is around 8kbp. However, I did not transfect successfully. My subject is Penicillium marneffei.

Seems like there is not much effect

for smaller size can go inside the cell

however too big size can increase viscosity and water pressure and and availability of Oxygen

Also in a paper of Chung et al. (1989) they compare different PEGs and find PEG3350 and PEG8000 as best working for E.coli

see links-

http://www.protocol-online.org/biology-forums/posts/26634.html

https://biology.stackexchange.com/questions/1995/why-is-peg-important-for-efficient-yeast-transformation