It is mentioned in the Hadith that the size of the Jannat of the very last man to emerge from Jahannam will be ten times the size of our earth.

Answer

There is absolutely no conundrum underlying the massive size of Jannat. Jannat despite its mind-boggling massive size will be an infinitesimal speck in the 'boundless' space created by Allah Azza Wa Jal. All of creation with its innumerable universes, is a speck in the vast 'limitless' spans of created-space.

The size of space cannever be measured nor ever be imagined by even Jibraeel (Alayhis salaam). Even the moron atheist scientists present mind-boggling sizes of stars which all are mere specks. Consider the size of the sun. They say that the sun is 1.3 million times the size of the earth. The largest star hitherto discovered is called Stephenson 2-18. This star of mind-boggling size is about 10 billion times the volume of the sun.

10 billion multiplied by 1.3 million equals about 20 quadrillion earths. There are billions of stars of such magnitude as this Stephenson star. The mind cannot imagine how huge is even this star which is a speck in space. They say that the two closest stars are separated by a distance of 25 million miles. When a person of Imaani intelligence looks at the sky and observes the stars, he is compelled to say as the Qur'aan mentions:

"O our Rabb! Indeed You have not created all of this in vain."

This furthest known star from the earth is at a distance of 19,000 light years. That is: the distance light travels in 19,000 years. The speed of light is 300,000 kilometres per second. In one minute, light travels 18 million kilometres. Thus the distance of the furthest star from the earth is absolutely mind-boggling. It defies imagination. In fact, in relation to this reality, even fiction ceases to be fiction and the reality of the distance seems 'fictitious' when in fact it is the truth testifying to the Qudrat of Allah Azza Wa Jal.

And above all, this furthest star of such vast dimensions is no where near to even the first Sama' (sky).

The stupendous size of these massive stars more than adequately answers the question on the size of Jannat. Beyond the furthest star visible in the most powerful telescope is only darkness. All and everything which the most powerful telescope makes visible are mere specks in the vast spanseless space created by Allah Azza Wa Jal. And, all of these mind-boggling specimens of creation have been created by a simple pronouncement of "KUN!" (Be!) The Qur'aan Majeed states:

"When Allah intends to create anything, He only says: 'Kun!', and it comes into existence (instantaneously)."