I too am in the early months of using the SharePoint (Online) REST APIs and on the whole are liking the experience.
A browser add-in I've found very useful to expand the results of and debug REST API calls is http://restclient.net/
Once I know I've got a successful call, I've found it straightforward to drill down into the results using the d.results[n]
syntax
Update:
So, I've done some more research now I see you are working with a document library and looking for document properties...
I turns out that the REST URL is slightly different for querying document properties: https://mysharepointsite/sites/site_name/_api/web/getfilebyserverrelativeurl('/sites/site_name/Document Library Name/File Name.suffix')
For me, this query returns the following fields:
"CheckInComment": "",
"CheckOutType": 0,
"ContentTag": "{9874C2F2-DB89-4604-BD13-32E30F5176D2},2,5",
"CustomizedPageStatus": 0,
"ETag": "\"{9874C2F2-DB89-4604-BD13-32E30F5176D2},2\"",
"Exists": true,
"IrmEnabled": false,
"Length": "9117",
"Level": 255,
"LinkingUrl": "",
"MajorVersion": 1,
"MinorVersion": 0,
"Name": "lifing hazard.jpg",
"ServerRelativeUrl": "/sites/dev/Skills Training Images/lifing hazard.jpg",
"TimeCreated": "2014-03-17T00:30:10Z",
"TimeLastModified": "2014-03-17T00:30:53Z",
"Title": "Excessive Weight",
"UIVersion": 512,
"UIVersionLabel": "1.0",
"UniqueId": "9874c2f2-db89-4604-bd13-32e30f5176d2"
where 'Length' is the file size in bytes.
MSDN Source - https://msdn.microsoft.com/en-us/library/office/dn450841.aspx