http://techdocs.altium.com/display/ADOH/Design+Rule+Checking
http://techdocs.altium.com/display/ADRR/PCB((Design+Rules+Reference))
Elecrow Design Rules
Silkscreen Text
Min Width 0.15mm / 6mil
Min Height 0.8mm
Vias and Traces
Min Clearance 0.15mm / 6mil
Min Via size 0.3mm / 12mil
Min Ring size 0.15mm / 6mil
Max Drill size 6.3mm / 248mil (otherwise holes will be milled)
Min Trace width 0.15mm / 6mil
Min Hole-to-Hole Clearance 0.254mm / 10mil
**note** The images displayed have the units of mm, to change between imperial and metric, while on your PcbDoc file with no other window open, press "q" on the keyboard
On your .PcbDoc file go to Design→Rules
A window should appear like this.
To access clearances, expand the "Electrical" tree menu, then expand the "Clearance" sub-tree. Click on the "Clearance" item in the subtree.
The table shows the clearance relationships between each object. You can change each value as you wish. But if elecrow is being used as the fabrication service, use the specified values above (or on their website for updated information) or higher for minimum clearances.