Hi repdetect,
Welcome to the forum 
The way to do this is to use the "Convert" facility which is accessed when an image is in the queue and you right click over the preview and choose "Convert Images".
There are a number of things to check before you do this.
First, make sure Interpolation is set to Fusion: - Job Properties below the preview.
Also check the "Antialias when DOWN sampling" setting by clicking the blue dot next to "Interp".
The default is Medium but I normally use "Low" which gives a sharper looking result. It depends on the subject matter; something like roof tiles will require a higher setting to avoid moir patterns.
With the image in the queue, open the Convert dialogue (right click menu), set to tiff, any colour conversion required, output folder file naming etc.
See screen shot attached below.
Next put a dot in the "Add to/replace existing filters" option, the filter dialogue will open, see second screen shot.
At the bottom of that window, set the image resolution (pixel size) you require; for your requirement it'll be a 1/3rd to 1/6th of the original.
Click Done and then OK on the Convert window.

Come back if more explanation is required.

Terry

Following on from my original answer to the query, and "What's the best way to do this in qimage and preserve highest quality?"
The best way is to scan at the desired final resolution in the first place. What size print do you want to make? Work back from that to get a pixel size; QU will give an excellent print with a 200ppi print size resolution and 300/360ppi is plenty.
Terry

Yes, small negatives, 35mm.

Your research was correct, 7200dpi scans don't show any increase in quality but there is a big BUT (no pun intended)

According to tests by the German site ScanDig (available in English) the scanner I use, a Plustek 7600i (and most other 7200dpi scanners), the effective resolution of a 7200dpi scan at 24 bits is only 3250ppi! but results in a file that's 70Mb+. The actual pixels at 3250 is 14Mb. The additional pixels in the 7200dpi scan are duplicates.
Experience has shown that a 3250dpi scan would suffice but unfortunately selecting the 3200dpi scan rate actually results in only 2600 dpi, unacceptable for me.
So the only way to get even 3250dpi is to scan at 7200 and throw away the duplicated pixels by resizing to about 14Mb.

Anyway here is a link to the review. Hope this helps.


BTW I love Qimage.

http://www.filmscanner.info/en/PlustekOpticFilm7600i.html

One other thought:
If you could live with the large file from the scan and print with that, the best quality would be obtained that way because only one interpolation would be done.
By downsizing and then QU interpolating to the printer native resolution, there are two interpolations. It's recommended that only one interpolation is used in the image to print process for best quality.
Terry

My last scan resulted in a 315Mb file too much to archive.

Hmmmm food for thought.

thanks much.