It is "look". This is a bare infinitive.
The relation between "make", "the floor" and "look bigger" is not a verb followed by a finite clause. Instead the verb make is followed by an infinitive verb clause, but it is a "bare" infinitive, without "to"
So at least one parsing is that "the floor" is the object of "make" and the infintive phrase is "look bigger". It is probably also possible to parse "the floor look bigger" as an infinitive clause with "the floor" as its subject.
Here is the parsing by computer:
(S (NP Big tiles)
(NP the floor)
(It will parse the other version, but only by assuming that "looks" is the plural of a noun "a look", which is obviously nonsensical)