Actually you can't, you can cut down most things but you can't get previews in preference to icons and you can't get more than two rows (from that menu, you have to go to about:config and browser.newtabpage.activity-stream.topSitesCount [for some reason the browser.newtabpage.rows and ..columns settings are still present but appear to be unused]).

if you turn off the new activity stream you can get the old behavior back.

browser.newtabpage.activity-stream.enabled = false

browser.newtabpage.activity-stream.feeds.section.topstories = false

browser.newtabpage.enhanced = false

browser.newtabpage.enabled = true

seems to respect my 5 columns and 3 rows with large preview icons.

I have all those already; it respects my setting 24 icons (4 rows of 6) rather than giving my prior 3 rows x 5 of page previews.

well I must have some other setting going on because i have 3 rows of 5 and the other setting is at 24 which both can't be true... 15 vs. 24 total.