Read and write block sizes of SQL server

Hi Experts,The data which are stored in pages are of 8 KB. When SQL server writes, does the block size vary (8KB, 64KB,128KB) and when it reads does it always read only in extents?For example : Assuming if a query will return worth of 50 MB , When SQL server fetch data from disk to buffer the rate of reads can it be calculated 50MB/64KB ?Similarly if it has write 50MB of data ,what will be the block size its going to write? please help me to understand this concept.