Shipbucket

Does anyone knows about the dimensions of its internal load space? (length X width x height)

Master of Wikipedia

Displacement:
340 tons (light)
415 tons (normal)
555 tons (full load)

should give an idea.

It can carry three main battle tanks (up to 150 tonnes), or ten armoured vehicles with 140 troops (up to 131 tonnes), or 8 armoured personnel carriers of total mass up to 115 tonnes, or 8 amphibious tanks or up to 500 troops (with 360 troops in the cargo compartment)

"The Zubr-class landing craft has a cargo area of 400 square metres (4,300 sq ft) and a fuel capacity of 56 tons."

That is from the Wikipedia page, and more what he wanted Heuhen.

The vehicle deck can hold 10x BTR-70 or 3x T-80B plus a detachment of infantry or 140 troops and 130 tons of cargo. Greek Zubrs can hold 8x M-113 or 3x Leopard 1A5.
Info from 16th Ed. of Combat Fleets of the World.

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As for internal height, I read that real Hellenic Army claimed that in can fit M270 MRLS, which is 2.62m tall (according to Military-Today) and Leopard I is 2.76m tall from same source. So, the internal height of Zubr vehicle deck must be as least 2.8m. Length of Leopard I is 7.1m (so we have at least 22m internal length, supposing some cm to spare among and front/rear of MBT's). 8 X M113 (probably in two "columns" of four each) we have ~19.5m tight.



Leopard I is 3.4m wide and left some space seems to remain clear, (while height seem tight, but Leo I still can fit).