What is the largest size butt implant that's safe to get and what's the safest placement?

 Make no mistake, it is indisputable that the best position (aka "placement") for buttock implants  is in the intramuscular plane not on top of the muscle and not "sub-fascial" which is exactly the same as on top of the muscle - so please do not be fooled by this terminology. Placing the  implants completely under muscle is not recommended because it risks injury to important structures such is the sciatic nerve.  
Other than placing buttock implants within the gluteus maximus muscle (aka “intramuscular”) NOT “subfascial”, the most important factors in creating the perfect derriere and/or hourglass figure with implants is indeed selecting the right size AND shape. I have found the best way to assure this is to simply measure the actual dimensions of the patient’s buttock cheeks and quantity and quality/density of the underlying gluteus muscles. With these anatomical measurements, I can select an implant that not only significantly augments the buttock but also supplies the correct 3-dimensional enhancement to look as real as possible...even in the most petite patients. Note, height and weight are considered but not nearly as relevant as the above measurements.The largest stock buttock implant currently available in the USA is a 750cc round implant. For Stanton Anatomic Oval implants, the largest stock size is 600cc. Indeed custom implants can be ordered larger, not only at a premium price but at added risk for most virgin patients (as compared to those already with implants and now upsizing, see below). Unlike breast implants which are placed under the very expandable tissue of the breast and half of the pectoral major muscle, buttock implants are placed entirely within the largest strongest muscle of the body, the gluteus maximus muscle, which is not nearly as expandable or forgiving. Thus most patients simply do not have a large enough native buttock muscle to fit an implant much over 500 or 600 ccs' without leading to significant complication such as wound breakdown/opening &/or implant exposure...necessitating removal. If a patient wants larger implants, 9-12 months after the initial implants have settled in to place and the muscle relaxed/stretched, then a second operation can be performed to replace them with larger ones (similar to the process of going bigger with breast implants). Hope this helps

The safest size implant greatly depends on your body size and type. The right surgeon will help you choose a size that best fits your body. Be sure to choose a Board Certified Plastic Surgeon who specializes in this procedure and is well respected in the medical community for it.