Query users in page increments using Sitecore PowerShell Extensions.

GetAllUsers-Pagination.ps1

$totalCount = 0
$page = 0
$pageSize = 25
$foundCount = 0
while(($users = [System.Web.Security.Membership]::GetAllUsers($page, $pageSize, [ref]$totalCount))) {
    Write-Host "Getting users"
    $page++
    $foundCount += $users.Count
    if($foundCount -ge $totalCount) {
        Write-Host "Done"
        break
    }
}