The opening two lines state the poem's premise. Put most literally, the poem is about the speaker needing help setting up a new house. Specifically, the speaker needs another person there to hold onto the other end of a tape measure while the speaker goes around and measures various parts of the house—likely in order to get furniture and decor that'll make the empty rooms feel more homey. Notice how the opening line itself is stretched by enjambment ("single span / requires a second ..."); it's as if it, too, is "greater than a single span" and as such requires a "second pair of hands"—or, rather, a second line in the poem—to be completed.
There is, of course, a whole world of meaning beyond this literal set-up, as the poem will go on to explore the enduring power of the relationships between mothers and their children. Even in this first stanza, though, it's clear that the speaker is still leaning on their mother for help despite being old enough to have a house of their own (note that we're using "they" for the speaker throughout this guide because no gender is mentioned in the poem).
The speaker's mother, for her part, appears in the poem almost as soon as the speaker makes it clear that her help is needed (the first line of the poem is in fact a moment of apostrophe). Given their apparent closeness, it's fair to assume that the speaker grew up with their mother in the family household. Perhaps this is the speaker's first time living away from home, and perhaps that's part of why the mother can be so helpful in this situation—she has experience of setting up a new house, and of turning a house into a home.
In lines 3 and 4, the speaker goes on to enumerates all the different things that need to be measured:
... windows, pelmets, doors,
the acres of the walls, the prairies of the floors.
It's a daunting task for the speaker, with the length of the list of items to be measure showing how much there is to be done. Furthermore, the lack of conjunctions like "and" in this sentence—a device technically called asyndeton—demonstrates the way in which the process feels almost endless. Indeed, the speaker exaggerates the size of the task by using metaphor too: "acres" and "prairies" relate to measurements of land, not walls. Accordingly, they relate to much larger distance than the ones actually being measured in this poem—but it helps relay the way that the speaker feels relatively out of their depth, metaphorically not yet ready to relinquish their mother's comforting support.