Only way coming to my mind is to go into the ccd_simulator.cpp and change the max value of the SIM_XRES and SIM_YRES Number Properties in initProperties().
bool CCDSim::initProperties()
{
// Most hardware layers wont actually have indi properties defined
// but the simulators are a special case
INDI::CCD::initProperties();
IUFillNumber(&SimulatorSettingsN[0],"SIM_XRES","CCD X resolution","%4.0f",0,4096,0,1280);
IUFillNumber(&SimulatorSettingsN[1],"SIM_YRES","CCD Y resolution","%4.0f",0,4096,0,1024);
IUFillNumber(&SimulatorSettingsN[2],"SIM_XSIZE","CCD X Pixel Size","%4.2f",0,60,0,5.2);
IUFillNumber(&SimulatorSettingsN[3],"SIM_YSIZE","CCD Y Pixel Size","%4.2f",0,60,0,5.2);
IUFillNumber(&SimulatorSettingsN[4],"SIM_MAXVAL","CCD Maximum ADU","%4.0f",0,65000,0,65000);
IUFillNumber(&SimulatorSettingsN[5],"SIM_BIAS","CCD Bias","%4.0f",0,6000,0,10);
IUFillNumber(&SimulatorSettingsN[6],"SIM_SATURATION","Saturation Mag","%4.1f",0,20,0,1.0);
IUFillNumber(&SimulatorSettingsN[7],"SIM_LIMITINGMAG","Limiting Mag","%4.1f",0,20,0,17.0);
IUFillNumber(&SimulatorSettingsN[8],"SIM_NOISE","CCD Noise","%4.0f",0,6000,0,10);
IUFillNumber(&SimulatorSettingsN[9],"SIM_SKYGLOW","Sky Glow (magnitudes)","%4.1f",0,6000,0,19.5);
IUFillNumber(&SimulatorSettingsN[10],"SIM_OAGOFFSET","Oag Offset (arcminutes)","%4.1f",0,6000,0,0);
IUFillNumber(&SimulatorSettingsN[11],"SIM_POLAR","PAE (arcminutes)","%4.1f",-600,600,0,0); /* PAE = Polar Alignment Error */
IUFillNumber(&SimulatorSettingsN[12],"SIM_POLARDRIFT","PAE Drift (minutes)","%4.1f",0,6000,0,0);
IUFillNumber(&SimulatorSettingsN[13],"SIM_ROTATION","Rotation CW (degrees)","%4.1f",-360,360,0,0);
IUFillNumberVector(SimulatorSettingsNV,SimulatorSettingsN,14,getDeviceName(),"SIMULATOR_SETTINGS","Simulator Settings","Simulator Config",IP_RW,60,IPS_IDLE);