What size fasteners should I use to mount fence bases to concrete?
A steel plate like you describe will presumably have some type of post or support attached to it. That attachment will be able to place a whole bunch of torque on the plate and the last thing you want is for the plate to start pulling loose when some chincey and cheap anchor screw starts to pull out of the concrete.

I would recommend the larger size if you are indeed going to be using the direct screw in type of concrete anchor. Be aware that the screw in concrete anchor does require an accurate hole diameter and deeper penetration into the concrete and larger diameter lead to better holding power. Follow instructions carefully on how this type of anchor is installed. 

If you have a situation where:


You have a hard time drilling holes into concrete and keeping the hole diameter from getting enlarged too much.
You do not need to be close to the edge of the concrete.


...then you may want to consider some quality expansion type anchors instead. Good ones will set you back cost wise even more than the larger screw in anchors that you linked to.