Here is a link that will help you determine the max size, based on your OS version: What type and size Media Card can I use in my device? - BlackBerryFAQ
SanDisk seems to be a popular manufacturer form Micro SD cards, but I have used a variety of different brands in my BlackBerrys and haven't had any issues. I would avoid buying form e-bay or Craig's Lists, though...I have heard that some sellers at these sites can sell some pretty shady stuff. Stick with retailers that you trust.