It would depend on the star's luminosity rather than its size, which in turn would depend on the star's surface temperature as much as the star's radius. Perhaps SpaceEngineer can provide the exact formula.

To know if a star could be seen during daylight we would have to calculate the star's apparent magnitude (I don't know the formula to calculate it from the star's radius, temperature and distance) and compare it with the apparent magnitude that is visible to the human eye during daylight (which I don't know either).**Added** (22.11.2011, 22:14)

---------------------------------------------

I've found the formula for the luminosity of a star:

L=4*pi*R^2*sigma*T^4

Where:

R is the radius of the star

sigma is the Stefan-Boltzmann constant 5.67×10−8 W·m-2·K-4

T is the surface temperature of the star

And here's the formula for a star's apparent magnitude:

m_star=m_sun-2.5*log10((L_star/L_sun)*(d_sun/d_star)^2)

Where

m_star is the apparent magnitude of the star

m_sun is the apparent magnitude of the sun

L_star is the luminosity of the star

L_sun is the luminosity of the sun

d_star is the distance to the star

d_sun is the distance to the sun

In order to be visible during daytime, the star's apparent magnitude would have to be lower than -2.5.

Check my Space Flight Simulator for Android!