Actually, the size specifications for fans is the length in mm of one SIDE of the square fan frame, and that typically is like 90, 120, or 140 mm for case ventilation fans. The spacing of the mounting HOLES on one side will be less than each of these, of course, but will need to match the spacing of holes in your case's front and rear fan locations. In your photo it APPEARS to show (NOT completely clear) at the rear location that there may be space and pre-drilled holes for EITHER a 120mm or 140 mm fan.
At the front you say you were able already to remove that lower front panel and find a fan space with pre-drilled mounting holes. So that's where you mount a front intake fan. VERY often the fan goes on the INSIDE of the frpotn panel. The grille in the panel you removed may be a fine mesh to act as a dust filter, OR it may have coarse mesh holes with a thin foam sheet behind it for the filter. Either way, from time to time you must remove that panel / filter and clean off accumulated dust to keep it free for air flow through.
MOST fans you buy come with arrows moulded into the outside of the fan frame. One points AROUND the frame to indicate blade rotation direction. The other points THROUGH the frame to tell you the AIR FLOW direction. When you go to mount each fan, you simply make sure it is turned to make that air flow go the direction you want.
A hint. Most fans with plastic frames have the holes in the four corners but these do NOT have screw threads cut into them. They do come with self-tapping screws. Normally you place the fan against the metal mounting plate with holes liked up then insert a screw from the other side of that plate, through the plate hole and into the fan frame hole. Then you need to turn the screw in with a fair amount of torque force to let the screw cut its own new thread into the hole. If you ever remove that fan and re-mount it, the threads will already have been cut so the torque force is much less.
You want fans for case ventilation. First choose the fan SIZE as above. Then in the specs for various fans look for two items. First is max AIR FLOW usually in CFM (cubic feet per minute), but sometimes in cubic metres per hour. Ideally you want more CFM, although you do NOT need the ultimate - VERY high CFM is not needed usually and means much more fan noise and power use. If you have a choice (according to the space available in each case mounting location), 140 mm fans normally can deliver more air flow at max speed that 120's, and hence in real use may run slower and more quietly for the same air flow. Second item is fan PRESSURE. For these case vent uses you need ones with optimal design for best air flow, not max pressure. So pressure ratings over 1.5 mm of water are NOT needed. LAST item to consider is noise in dBA - less is better, but make sure you get the right max air flow first.
Last characteristic is PINS. Older fans have cables and a connector with THREE wires and holes in the end connector. They are called Voltage Control Mode fans or just 3-pin fans. The newer design has 4 wires and holes, called PWM Mode fans or (surprise) 4-pin fans. Most modern mobos can use 4-pin fans and they are a better design IF you rmobo is built for them. If you need advice on what your mobo can use, tell us the maker and exact model of your motherboard.