I found following my self:
2.1.1 Page size and sector size
The sector size of the LPC5500 series internal flash memory is 32 KB and the page size is 512 bytes.
The minimum programming size is the page size, 512 bytes.
The minimum erase size is page size, 512 bytes
AN12949 where I found this info is attached.
This seem to be quite large potion - so if there is any tip to program smaller potion? then I like to know.
Thanks,
Jakob