Here are the specifcation of the lens. On 350D, the sensor is APS-C 22mm x 15mm.
Are the following calculations correct?
1) At 300mm, the mag is 0.26, so the smallest square to fill the frame is 85mm*58mm (22/0.26, 15/0.26);
2) At 300mm with EF25mm, mag is 0.39, the smallest square to fill the frame is 56mm*38mm;
(22/0.39, 15/0.39);

On Tamron 90mm Macro. I have 1:1 mag, the smallest square will be 22mm x 15mm.
With EF25 attached, I can shoot a square of (22/(0.39/0.26) = )14.7mm X (15/(0.39/0.26) = )10mm.

EF 70-300 IS USM.
Angle of view (horizontal, vertical, diagonal): 29 to 6 50', 19 30' to 4 35', 34 to 8 15'
Lens construction (elements/groups): 15/10
No. of diaphragm blades: 8
Minimum aperture: 32-45
Closest focusing distance: 1.5 metres
Maximum magnification: 0.26x (at 300mm)
Distance information: Yes
Image stabilizer: 3-stops
AF actuator: Micro USM
Filter diameter: 58mm
Max. diameter x length: 76.5 x 142.8mm
Weight: 630g
Magnification with Extension Tube EF12 II: 0.32x to 0.04x
Magnification with Extension Tube EF25 II: 0.39x to 0.09x
Lens hood: ET-65B
Soft case: LP1222
Extenders EF1.4x II and EF2x II: Not compatible