A couple of ideas.
1) try to make certain you can see the whole table on your screen before you highlight and copy.
2) When you paste into PowerPoint, use Paste Special/picture or Paste Special/picture (enhanced metafile). When you copy and then Paste a piece of Excel into PowerPoint, you are embedding a copy of this Excel spreadsheet into PowerPoint. It will allow you to go to the PowerPoint file directly to change some numbers in the table, but sometimes the Paste doesn't work so well, and forces you to do a bunch of workarounds to get the table to look like you want.
3)Of course if it's just a table you want to copy, why not create a table in PowerPoint, then copy the cell contents of the Excel worksheet to your slide?