Not the prettiest thing in the world but gets the job done by printing each user's mailbox size & total items into a HTML table:

get-mailbox -database "Mailbox Database 1706745955" | get-mailboxstatistics  | where {$_.objectclass -eq "Mailbox"} | sort-object totalitemsize -descending | select-object @{label="User";expression={$_.DisplayName}},@{label=
"Total Size (MB)";expression={$_.TotalItemSize.Value.ToMB()}},@{label="Items";expression={$_.ItemCount}}  | convertto-html | out-file report.htm

May need to modify the mailbox database name.