While "Opera Mini" is primarily a mobile browser, older desktop versions (often mistaken for it) are available in portable formats. If you are specifically looking for the classic "Presto" engine version roughly matching your criteria, Opera 12.18 Portable is the most common legacy download. Legacy Portable Opera Downloads Opera 12.18 Portable (Legacy)
Focused on basic web access for Java-based phones. Version 4.0 (2007) was a "rewrite" that introduced the ability to view webpages as they appeared on a PC, rather than just mobile-optimized versions.
This version was roughly 8.5 MB to 12.1 MB depending on the specific build and chipset (ARM, MIPS, or X86). It is highly sought after for its inclusion of the Presto engine , which was faster and more compatible with older web standards than newer versions. You can find archived APKs on APKMirror .
If you are looking for the desktop "Portable" version (often around to 133 MB download size), it is designed to run from a USB drive without installation.
Select under the "Install for" section to ensure it remains portable.