Size and size_t in dsa API

Hi, 

I'm trying to use DSA API and confused a little bit about $subject.

Some type of return value or arguments are defined as size_t
but others are as Size.

Example: 
- dsa_area *dsa_create_in_place(void *place, size_t size,
                    int tranche_id, dsm_segment *segment)

- Size dsa_minimum_size(void)

I'm confused because Size and size_t is virtually equal but 
they are mixed in dsa.c. At first I was thinking that there is
some distinguishing convention to use based on some context.

But according to this thread, which seems gone anywhere,
they are same. 
https://www.postgresql.org/message-id/CAEepm%3D1eA0vsgA7-2oigKzqg10YeXoPWiS-fCuQRDLwwmgMXag%40mail.gmail.com 
Are there still someone trying to this?
As a non-expert developer's opinion, I think mixing of Size and size_t makes difficult to understand source code.

===================================================
Takeshi Ideriha
Fujitsu Limited