Choose type of racking?
It is very important to know what pallet size and weight you are using now and anticipate the future in order to protect your investment.
If you under estimate the weight of the pallets, you either will loose one pallet space on each level or have the risk to have the pallet racking collapsing.
Be over cautious and you will over spend.
The minimum space recommended between pallets or between a pallet and the frame is 75mm. However this dimension will also depend of different factors (i.e. side-shift on forklift, width of the aisle between racking, driver experience, stock rotation…)
The most common size pallet is 1200mm x 1000mm, another popular size is 1200mm x 800mm. However you can find other sizes, also sometimes the goods on the pallets overhang and must be accounted for.
On a general note if you use only 1000mm pallet and the load is flush with the pallet it will be more space efficient to use 2250mm beams(75+1000+75+1000+75=2225mm), or 2550mm if the load overhang.
If you are using 800mm pallets or a mixture of 800 and 1000mm wide pallet, it will be recommended to use 2700mm beams, allowing you to store 3 pallets 800mm on one beam (75+800+75+800+75+800+75=2700mm).
Regarding the height:
I would recommend to my customer to use all the height available to them, as it is free!
If the frame height you are choosing now is only a fraction of the total height available to you, it might be costly to extend or replace them when you have outgrown your storage capacity. If the reason why you choose a small frame is the reach of your forklift, it is always easy to trade-in your mast or your forklift for a higher reach one and add extra beams when required, than to extend or get another warehouse or having to empty your existing one to replace the frames.