How to set custom column widths in material ul?

Question

material design

How to set your own column width in material ul if the proposed options do not fit the layout?
Need Navbar to be 275px, Content 600, Sidebar 350px

<Container>
        <Grid container spacing={5}>
          <Grid item xs={2}>
            <Navbar />
          </Grid>
          <Grid item xs={6}>
            <Route path='/' component={HomePage} />
            <Route path='/user/:id' component={UserPage} exact />
          </Grid>
          <Grid item xs={4}>
            <SideBar />
          </Grid>
        </Grid>
      </Container>