To get the best print quality possible, it's best to limit the print size to 300 PPI for your input file. So if you scan a 35mm slide at 4000 PPI, and you have a 35mm slide that is 1.4" on the long side (36mm x 24mm), then you will have about  3360 pixels on the long side. Divide that by 300 and you have 11.2". This being said, we have printed slide scans as large as 24" wide with good results after up-sampling the file, but 35mm slides in particular are limited in what they can produce. Larger film scans, such as medium format film and 4x5 transparencies obviously can produce larger prints, but the math is the same.