According to YEC, were the days of Genesis 1 all 24 hour periods?

Yet, YECs hold that the Earth was quite different back then than it is now.

Depends on what you mean by "quite different". Different climate, yes. (In the sense that China and Russia have different climates. Not in the sense that Venus and Mars have different climates!) Different topography, continent(s) (in particular, only one before the Flood), yes. Different axial tilt, perhaps. (To be clear, all of the preceding are beliefs, but aren't 100% certain.) Different rate of rotation? Well... "probably"¹, but also most likely not significantly different. Days might have been 23.5 hours; they weren't 2.4 hours or 240 hours.

(¹ In fact, certainly, in the sense that Earth's rate of rotation today differs from a decade, or a century, ago. But perhaps more different than merely back-plotting the current changes ~6ky.)

Indeed, the sun doesn't even appear to be created on the first days of Genesis.

Correct. However, the Sun is not needed; only a source of light, which was Created right away (Genesis 1:3), and which "comes and goes" regularly, as observed from some particular spot. The latter is readily achieved by the light being directional (at least, relative to Earth; so, directional in the sense that the Sun is directional) while the Earth rotates.

If conjectures about pre-Flood Earth having a single continent are correct, the idea that Genesis would consider only the land as its point of reference is perfectly defensible. Even if not, God's focus is ultimately on Eden, and it's reasonable to assume that "evening" and "morning" refer to this reference point.

If so, a day can't be defined as the period of time from when the sun rises to when it rises again.

Pedantically correct, but irrelevant, because...

So what is the primary definition of 'day' according to YECs - is this a 24 hour period?

...a "day" refers to the Earth's rotational period, or more specifically, its cycles of light and dark ("evening and morning"), which, per above, don't require a Sun.

As noted, "24 hours" is an approximation. It's an approximation today, albeit the true value is extremely close. At Creation, a day might have been slightly longer or shorter, though not significantly so; the length of Earth's day is one of many, many values which has been carefully tuned to make life possible.

In fact, this science may be our best evidence that the "days" in Genesis 1 were (roughly) 24 hours... because if they weren't, something very, very different must have been going on in terms of Earth's energy cycle. One would have to argue, therefore, that the length of a "day" changes dramatically between Genesis 1:13 and 1:19. It seems far more likely that God, having a plan beforehand, would have arranged for the light of 1:3 to have similar energy input as the Sun which He subsequently Created on day 4.

and if so, how do they know that is what Genesis means?

The use of "evening and morning" attests to "one Earth rotational period". Moreover, the six-plus-one days of Genesis are the basis of the week, as attested multiple times in the Bible. This idea of a week, or the various subsequent attestations to Creation taking six days, make no sense if "day" meant something different during the events of Genesis 1. To an extent, the word "day" (as used in the Bible, anyway) only makes sense to the extent it is defined in Genesis 1. Moreover, since a "day" is one light-dark cycle, science tells us that a "day" can't vary significantly from 24 hours without Earth's energy cycle being very different from its present state.

In summary, any alternate meaning either contradicts the textual definition of "day" as "evening and morning", or requires substantially different astronomical conditions. Since the latter is at least as miraculous as what is suggested by a plain reading of Genesis 1, and additionally makes a mess of the text, such explanations are extremely unlikely.