## 2 Experimental Setup  
The experiment was performed at the X-ray Pump Probe instrument [11] of the Linac Coherent Light Source [1]. The incident X-ray FEL pulses were monochromatized at 9.83 keV using a C\({}^{*}\)(111) monochromator [12]. We controlled the focused beam size at the sample plane by a combination use of slits and X-ray focusing optics, i.e., beryllium compound refractive lens (CRL) with a \(\sim\)3.85 m focal length. In the focusing geometry, the slits both upstream and downstream of the CRLs were used to define the numerical aperture of the beam entering the lens and clean up upstream small-angle scatterings. The slits moreover mitigated the decoherence effect from the monochromator diamond crystal surface miscut [12; 13]. At the sample position, the beam profile was characterized using a high-resolution microscope (Nikon CFI S Plan Fluor 20\(\times\)) imaging a 20 \(\upmu\)m Ce:LuAG scintillator screen. With the upstream slit sizes set to 50 and 100 \(\upmu\)m, the full-widths at half-maximum (FWHM) intensity of the focal spots were measured to be 10 and 5 \(\upmu\)m, respectively. The beam size can be enlarged by either translating the lens stack longitudinally to position the sample out of the focus or by removing the lens and using only the slits.  
In total, we acquired speckle patterns under nine different scattering conditions: four were realized by varying CRL focusing conditions, four by using slits opening of different sizes, and finally one in the unfocused condition with slits fully open as a baseline measurement. For the four measurements with focused beams, the 5 and 10 \(\upmu\)m conditions have been mentioned above, while the other two (18, 38 \(\upmu\)m) were achieved by translating the lens stack so that the sample was 70 mm downstream and 230 mm upstream of the focal position, respectively. Silica nanospheres contained inside a quartz capillary with a nominal inner diameter of 1 mm were used as the scattering sample. An ePix100 detector (pixel size, 50 \(\upmu\)m) [14; 15] was placed \(L\approx 9.5\) m downstream of the sample to record the small-angle coherent scattering. The detector has a gain non-uniformity of around 5% which is software-corrected to be around 0.5%.  
When adjusting the beam size at the sample location using the slits or focusing optics, the sample diffraction volume changes accordingly, leading to a change of speckle size at the detector plane. For example, displayed in Figure 1b,c are the speckle patterns in the two focusing conditions mentioned above, where the speckle size is visually shrunken when the focal spot size expands by a factor of 2. On the other hand, the detector pixel angular resolution was fixed, and this therefore led to a change in the contrast levels in the measured speckle patterns [16] as plotted in Figure 1d, where the oversampling ratio is defined as the speckle size (FWHM) normalized by the detector pixel size. Here, the theoretical calculation (gray curve) following Ref. [16] and the simulation (red dots) following Ref. [17] agree well with each other. It is also worth noting here that this plot only shows the contrast reduction factor from the oversampling ratio, whereas many other parameters including the beam coherence properties may also change when adjusting the beam sizes and contribute to further contrast changes.