1.5 km is probably a good choice as a compromise. But you should be aware of what you're compromising. An Italian infantry battalion could defend about a 1,000 meter front or a 1.0 Km hex, the size you started with. Yet the combat range of such a battalion was about 450 meters (As evidenced by the 45mm mortar and the range of 7X military binoculars.). About the only infantry battalion weapon that exceeded this range was the 81mm mortar and with which, on level ground, you cannot spot your hits. So while your hex was correct for battalion size it was off by 50% for battalion range plus the terrain errors that come with such a small scale.
By increasing your hex size, you decrease you terrain error but now increase your range error. I would solve this problem by giving an Italian infantry battalion a combat range of ZERO. It has defensive firepower only. Technically, we're describing a WW1 army. In WW1 the attackers charged the other's defensive trenches and machineguns. That was the only way to get close enough to fight. Back then, the opposing trenches were about 1 Km apart and, without the charge, neither side could hit the other, giving each side a combat range of ZERO using 1 Km hexes. In WW1, not many made it to the enemy to actually apply their defensive firepower (They had left their machineguns behind.).
In WW2, the Germans solved the problem of charging infantry with a combat range of zero by charging with tanks followed by infantry. Yet the infantry still had a combat range of ZERO until they reached the enemy. The previous 1 Km is now 0 Km. Now their defensive firepower can be applied to the enemy. The attacker brought their previously defensive weapons with them (motorized machineguns plus tank artillery guns) with them to the enemy line and the effect was dramatic.
Yet in WW2 the Italians were still living in WW1. An L3/33 tank could be shot to pieces by armor piercing bullets and an M13/40 taken out by anti-tank guns of which the British had no shortage. Unable to reach the British line, the Italians' firepower remained defensive only. When the Italians attacked Sidi Barrani in September 1940, they inflicted only 60 casualties, an attack factor of virtually ZERO. Why only 60? Because no motorized artillery or machineguns reached the British defenders. They had only rifles with an effective range of 250 meters. The British inflicted 500 casualties. Why? Because they could fire with full defensive fire power.
How do you cover this with your rules? Very simply. DEFENSE FIRES FIRST. So the British fired first at Sollom/Halfaya Pass with artillery and machineguns, inflicting 500 casualties. Now the surviving Italians fire second but only get 60 British with almost zero attack factors.
By WW2, rifles were useful only for defending machinegun emplacements. A battalion of them might produce 10 British casualties.
This is when we have to consider turn duration. Again, the enemy's turn begins as soon as it can give, and carry out, an order. In this case, the British made the decision to retreat. So the "IGO" turn began with the Italian attack and UGO began with the British retreat. The British had three possible orders. They could counterattack, retreat, or maintain their position. Had they counter attacked, they become the attackers, the Italians the defenders, and DEFENSE FIRES FIRST. The result is way more British casualties as the Italians are bringing up motorized artillery, mortars, and LMG's. The British wisely decided not to do that. They could have maintained their position but their troops at Sollom would be cut off if the Italians took Halfaya Pass. So Sollom had to retreat. If they decided to stay and and fight without counterattacking, they would remain the defenders and the Italians the attackers and, once again, DEFENSE FIRES FIRST. That would have been ugly for Italy with another 500 casualties.
When the British retreated, their UGO turn ended when they reached Azziziya on September 15, a turn of three days. The Italians did not frontally attack the British here but moved south of them on a dirt road in two columns with nothing between them and Sidi Baranni (11th Hussars was keeping the road to Maktilla open.). The Italians reached Maktilla on their second turn (September 17). The Italians used five days in two turns or 2.5 days per turn, or basically the same three day turn as the British (The Italians would have used six days if they had gone further than Maktilla, stopping on September 18, but elected not to.). The British also carried three day's ammunition, another reason to use three days. However, three days was the response time for 5,000 men and not 750-1100.
So when you use 1.5 Km hexes, adjacent enemy units do not have to fight. There is a "no man's land" between them. One side has to elect to attack (charge), meaning he's trying to enter the other hex, and then the other side fires first. If the defender retreats, the attacker automatically takes, and moves into, the vacated hex (Also, tanks never defend.).
This will solve the problem of where your scale in defense does not match your scale in offense. Once you calculate a turn duration, your results should be very accurate. I'd be interested in learning the turn time you come up with for battalions.
If you want to know how the Italians can win under these rules, ask.
