False "Object does not fit into print volume" warning from OctoPrint
That's the purging that Slic3r PE adds, the broad line of filament at the edge of the sheet. That is outside the official print volume, which triggers this error.

The G-Code generated by Slic3r PE at the start of the file contains the following lines:

G1 Y-3.0 F1000.0 ; go outside print area
G92 E0.0
G1 X60.0 E9.0  F1000.0 ; intro line
M73 Q0 S174
M73 P0 R173
G1 X100.0 E12.5  F1000.0 ; intro line
G92 E0.0


You can see that it explicitly goes to -3 on the Y axis, and then extrudes two times on a line along the X axis.

The model size detection is labeled as beta in the settings dialog. It's not very reliable at detecting stuff like this purging line outside the boundaries.