You can't specify a size for each backup file when backing up to multiple files but you can estimate their size based on the db size. So for a 200GB db spread across 4 files, they should all be roughly 50GB each. For example, if you have 100GB free on the C drive and 50GB free on the D and E drives you could do something like this:
BACKUP DATABASE [dbname] TO
DISK = N'C:\dbname1.bak',
DISK = N'C:\dbname2.bak',
DISK = N'D:\dbname3.bak',
DISK = N'E:\dbname4.bak'
GO