Additional current source for large LED-7-segment-display
That kind of large digit display uses many LEDs in series-parallel. You need more voltage than the ICM7218 can provide- in particular the forward voltage for each segment is 8V typical, 10V maximum. Thus you'd probably want at least 12V, and preferably 15V to drive the LEDs (note carefully that the decimal point has fewer LEDs and series and does not have two parallel strings so it has less forward voltage for the same brightness). Higher voltage than the absolute minimum is preferably to allow the use of simpler source and sink drivers and to allow a bit more resistance (or compliance for a constant-current circuit, as the 7218 uses) so the brightness variations between segments and digits would not be a problem. 

While it would be possible to add boosting and external current control to the 7218, you could run into problems with ghosting and it would not be a simple circuit. 

I think you're better off to start from scratch with a 15V or higher supply and darlington array or MOSFET drivers. 

Here is a much better datasheet direct from the manufacturer. 



One simple way to drive four large digits like this would be to use 74HC595s (four of them daisy-chained to drive four digits) and four ULN2803 darlington drivers (and 32 series resistors- one for each segment and a higher-value one for each decimal point). That is what is called "static drive"- the segments are on continuously, not multiplexed. The ULN2803 can handle up to 50V and can drive up to 50mA with all 8 outputs on in a fairly hot environment (based on the datasheet, it looks like they're intended to be used at about 20mA/segment). Total current would be maximum 20mA * 32 =  640mA @ 15V for four digits. 



That will work with the common-anode displays (as shown in the above display schematic). For the common-cathode ones, you'd have to use a 8-channel similar source driver UDNxxxx in place of the ULN2803.