This was with virtualbox 6.1.8 (from debian). I'll try the latest 6.1.10 and new install, wonder which version you are using? [nb: 6.1.0-6.1.6 have known resize-bugs].
This does of course raise the point -- if 1024x768 is the minimum, why is it booting into 800x600 at all ...
Mintreport fits in 1024x768, that's the minimum required resolution to run Mint.
It does resize but after a delay (5s if I remember well), and only in live mode. Tested with VMSVGA.
secondly -- 'mintreport' handles very badly smaller screens (e.g. in this case 800x600 default mode, but same issue applies to smaller netbooks and so-on)... I think it should have a smaller window and/or become 'adaptable' -- i.e. use a small mode when x resolution below a certain size...
It may just open with no reachable controls on any side of screen... 'mintwelcome' only just (sort of) fits, with the close button partly off-screen.
I think these annoyances matter as they may flummox new users, especially if you are in a 'video driver issue' or 'need to install guest addiitions' scenario. 1024x768 and 800x600 are not uncommon 'default vesa modes', the core system ought to (cope) with the latter, in my view.
firstly -- r.e. running in virtualbox and resizing -- would (seem) that this version doesn't manage to auto-resize on virtualbox at all, in any mode, which was working in earlier beta in VboxVGA or VboxSVGA modes.
I notice now installing open-vm-tools and alas I'm not sure this helps. Talking to #vbox this whole area of 'what guest additions installed' is a wider issue/mess, collaboration with redhat broken down last July when some developer passed away, and so-on!.
In short, currently virtualbox-guest-users will need to manually install addinions (which now auto-starts more easily etc.)
However, there still (seems) to be some sort of regression over previous beta's, not sure exactly whats' going on.